Bug 717283 - Use tiles on-demand. r=pcwalton, snorp

Instead of tying the tile-buffer in MultiTileLayer directly to the back-buffer
of the page, make sure rendering is always aligned to the tile grid and use
tiles on-demand. This makes better use of tiles when panning/zooming, and opens
up the route for further optimisations.

mobile/android/base/gfx/GeckoSoftwareLayerClient.java

@@ -80,9 +80,15 @@ public class GeckoSoftwareLayerClient extends LayerClient implements GeckoEventL
     private ByteBuffer mBuffer;
     private Layer mTileLayer;
 
-    /* The viewport rect that Gecko is currently displaying. */
+    /* The viewport that Gecko is currently displaying. */
     private ViewportMetrics mGeckoViewport;
 
+    /* The viewport that Gecko will display when drawing is finished */
+    private ViewportMetrics mNewGeckoViewport;
+
+    /* The offset used to make sure tiles are snapped to the pixel grid */
+    private Point mRenderOffset;
+
     private CairoImage mCairoImage;
 
     private static final IntSize TILE_SIZE = new IntSize(256, 256);
@@ -108,6 +114,7 @@ public class GeckoSoftwareLayerClient extends LayerClient implements GeckoEventL
         mScreenSize = new IntSize(0, 0);
         mBufferSize = new IntSize(0, 0);
         mFormat = CairoImage.FORMAT_RGB16_565;
+        mRenderOffset = new Point(0, 0);
 
         mCairoImage = new CairoImage() {
             @Override
@@ -117,8 +124,6 @@ public class GeckoSoftwareLayerClient extends LayerClient implements GeckoEventL
             @Override
             public int getFormat() { return mFormat; }
         };
-
-        mTileLayer = new MultiTileLayer(mCairoImage, TILE_SIZE);
     }
 
     public int getWidth() {
@@ -151,6 +156,8 @@ public class GeckoSoftwareLayerClient extends LayerClient implements GeckoEventL
 
         GeckoAppShell.registerGeckoEventListener("Viewport:UpdateAndDraw", this);
         GeckoAppShell.registerGeckoEventListener("Viewport:UpdateLater", this);
+        GeckoAppShell.registerGeckoEventListener("Document:Shown", this);
+        GeckoAppShell.registerGeckoEventListener("Tab:Selected", this);
 
         // This needs to happen before a call to sendResizeEventIfNecessary
         // happens, but only needs to be called once. As that is only called by
@@ -163,9 +170,9 @@ public class GeckoSoftwareLayerClient extends LayerClient implements GeckoEventL
         sendResizeEventIfNecessary();
     }
 
-    private void setHasDirectTexture(boolean hasDirectTexture) {
-        if (hasDirectTexture == mHasDirectTexture)
-            return;
+    private boolean setHasDirectTexture(boolean hasDirectTexture) {
+        if (mTileLayer != null && hasDirectTexture == mHasDirectTexture)
+            return false;
 
         mHasDirectTexture = hasDirectTexture;
 
@@ -173,6 +180,7 @@ public class GeckoSoftwareLayerClient extends LayerClient implements GeckoEventL
         if (mHasDirectTexture) {
             mTileLayer = new WidgetTileLayer(mCairoImage);
             tileSize = new IntSize(0, 0);
+            mRenderOffset.set(0, 0);
         } else {
             mTileLayer = new MultiTileLayer(mCairoImage, TILE_SIZE);
             tileSize = TILE_SIZE;
@@ -186,18 +194,62 @@ public class GeckoSoftwareLayerClient extends LayerClient implements GeckoEventL
         // Force a resize event to be sent because the results of this
         // are different depending on what tile system we're using
         sendResizeEventIfNecessary(true);
+
+        return true;
     }
 
-    public void beginDrawing(int width, int height) {
+    public boolean beginDrawing(int width, int height, String metadata, boolean hasDirectTexture) {
+        // If we've changed surface types, cancel this draw
+        if (setHasDirectTexture(hasDirectTexture))
+            return false;
+
         beginTransaction(mTileLayer);
 
+        try {
+            JSONObject viewportObject = new JSONObject(metadata);
+            mNewGeckoViewport = new ViewportMetrics(viewportObject);
+
+            // We only need to set a render offset/allocate buffer memory if
+            // we're using MultiTileLayer. Otherwise, just synchronise the
+            // buffer size and return.
+            if (!(mTileLayer instanceof MultiTileLayer)) {
+                if (mBufferSize.width != width || mBufferSize.height != height)
+                    mBufferSize = new IntSize(width, height);
+                return true;
+            }
+
+            // If the origin has changed, alter the rendering offset so that
+            // rendering is snapped to the tile grid and clear the invalid area.
+            boolean originChanged = true;
+            Point origin = PointUtils.round(mNewGeckoViewport.getDisplayportOrigin());
+
+            if (mGeckoViewport != null) {
+                Point oldOrigin = PointUtils.round(mGeckoViewport.getDisplayportOrigin());
+                originChanged = !origin.equals(oldOrigin);
+            }
+
+            if (originChanged) {
+                Point tileOrigin = new Point((origin.x / TILE_SIZE.width) * TILE_SIZE.width,
+                                             (origin.y / TILE_SIZE.height) * TILE_SIZE.height);
+                mRenderOffset.set(origin.x - tileOrigin.x, origin.y - tileOrigin.y);
+                ((MultiTileLayer)mTileLayer).invalidateBuffer();
+            }
+        } catch (JSONException e) {
+            Log.e(LOGTAG, "Bad viewport description: " + metadata);
+            throw new RuntimeException(e);
+        }
+
         if (mBufferSize.width != width || mBufferSize.height != height) {
             mBufferSize = new IntSize(width, height);
 
-            // Reallocate the buffer if necessary
+            // We over-allocate to allow for the render offset. nsWindow
+            // assumes that this will happen.
+            IntSize realBufferSize = new IntSize(width + TILE_SIZE.width,
+                                                 height + TILE_SIZE.height);
 
-            // * 2 because it's a 16-bit buffer (so 2 bytes per pixel).
-            int size = mBufferSize.getArea() * 2;
+            // Reallocate the buffer if necessary
+            int bpp = CairoUtils.bitsPerPixelForCairoFormat(mFormat) / 8;
+            int size = realBufferSize.getArea() * bpp;
             if (mBuffer == null || mBuffer.capacity() != size) {
                 // Free the old buffer
                 if (mBuffer != null) {
@@ -208,39 +260,35 @@ public class GeckoSoftwareLayerClient extends LayerClient implements GeckoEventL
                 mBuffer = GeckoAppShell.allocateDirectBuffer(size);
             }
         }
+
+        return true;
     }
 
-    private void updateViewport(String viewportDescription, final boolean onlyUpdatePageSize) {
-        try {
-            JSONObject viewportObject = new JSONObject(viewportDescription);
+    private void updateViewport(final boolean onlyUpdatePageSize) {
+        // save and restore the viewport size stored in java; never let the
+        // JS-side viewport dimensions override the java-side ones because
+        // java is the One True Source of this information, and allowing JS
+        // to override can lead to race conditions where this data gets clobbered.
+        FloatSize viewportSize = getLayerController().getViewportSize();
+        mGeckoViewport = mNewGeckoViewport;
+        mGeckoViewport.setSize(viewportSize);
 
-            // save and restore the viewport size stored in java; never let the
-            // JS-side viewport dimensions override the java-side ones because
-            // java is the One True Source of this information, and allowing JS
-            // to override can lead to race conditions where this data gets clobbered.
-            FloatSize viewportSize = getLayerController().getViewportSize();
-            mGeckoViewport = new ViewportMetrics(viewportObject);
-            mGeckoViewport.setSize(viewportSize);
+        LayerController controller = getLayerController();
+        PointF displayportOrigin = mGeckoViewport.getDisplayportOrigin();
+        Point tileOrigin = PointUtils.round(displayportOrigin);
+        tileOrigin.offset(-mRenderOffset.x, -mRenderOffset.y);
+        mTileLayer.setOrigin(tileOrigin);
+        mTileLayer.setResolution(mGeckoViewport.getZoomFactor());
 
-            LayerController controller = getLayerController();
-            PointF displayportOrigin = mGeckoViewport.getDisplayportOrigin();
-            mTileLayer.setOrigin(PointUtils.round(displayportOrigin));
-            mTileLayer.setResolution(mGeckoViewport.getZoomFactor());
-
-            if (onlyUpdatePageSize) {
-                // Don't adjust page size when zooming unless zoom levels are
-                // approximately equal.
-                if (FloatUtils.fuzzyEquals(controller.getZoomFactor(),
-                        mGeckoViewport.getZoomFactor()))
-                    controller.setPageSize(mGeckoViewport.getPageSize());
-            } else {
-                Log.d(LOGTAG, "Received viewport update from gecko");
-                controller.setViewportMetrics(mGeckoViewport);
-                controller.abortPanZoomAnimation();
-            }
-        } catch (JSONException e) {
-            Log.e(LOGTAG, "Bad viewport description: " + viewportDescription);
-            throw new RuntimeException(e);
+        if (onlyUpdatePageSize) {
+            // Don't adjust page size when zooming unless zoom levels are
+            // approximately equal.
+            if (FloatUtils.fuzzyEquals(controller.getZoomFactor(),
+                    mGeckoViewport.getZoomFactor()))
+                controller.setPageSize(mGeckoViewport.getPageSize());
+        } else {
+            controller.setViewportMetrics(mGeckoViewport);
+            controller.abortPanZoomAnimation();
         }
     }
 
@@ -248,17 +296,17 @@ public class GeckoSoftwareLayerClient extends LayerClient implements GeckoEventL
      * TODO: Would be cleaner if this took an android.graphics.Rect instead, but that would require
      * a little more JNI magic.
      */
-    public void endDrawing(int x, int y, int width, int height, String metadata, boolean hasDirectTexture) {
+    public void endDrawing(int x, int y, int width, int height) {
         synchronized (getLayerController()) {
             try {
-                updateViewport(metadata, !mUpdateViewportOnEndDraw);
+                updateViewport(!mUpdateViewportOnEndDraw);
                 mUpdateViewportOnEndDraw = false;
-                Rect rect = new Rect(x, y, x + width, y + height);
 
-                setHasDirectTexture(hasDirectTexture);
-
-                if (!mHasDirectTexture)
+                if (mTileLayer instanceof MultiTileLayer) {
+                    Rect rect = new Rect(x, y, x + width, y + height);
+                    rect.offset(mRenderOffset.x, mRenderOffset.y);
                     ((MultiTileLayer)mTileLayer).invalidate(rect);
+                }
             } finally {
                 endTransaction(mTileLayer);
             }
@@ -277,29 +325,28 @@ public class GeckoSoftwareLayerClient extends LayerClient implements GeckoEventL
         ByteBuffer tileBuffer = mBuffer.slice();
         int bpp = CairoUtils.bitsPerPixelForCairoFormat(mFormat) / 8;
 
-        for (int y = 0; y < mBufferSize.height; y += TILE_SIZE.height) {
-            for (int x = 0; x < mBufferSize.width; x += TILE_SIZE.width) {
-                // Calculate tile size
-                IntSize tileSize = new IntSize(Math.min(mBufferSize.width - x, TILE_SIZE.width),
-                                               Math.min(mBufferSize.height - y, TILE_SIZE.height));
-
+        for (int y = 0; y <= mBufferSize.height; y += TILE_SIZE.height) {
+            for (int x = 0; x <= mBufferSize.width; x += TILE_SIZE.width) {
                 // Create a Bitmap from this tile
-                Bitmap tile = Bitmap.createBitmap(tileSize.width, tileSize.height,
+                Bitmap tile = Bitmap.createBitmap(TILE_SIZE.width, TILE_SIZE.height,
                                                   CairoUtils.cairoFormatTobitmapConfig(mFormat));
                 tile.copyPixelsFromBuffer(tileBuffer.asIntBuffer());
 
                 // Copy the tile to the master Bitmap and recycle it
-                c.drawBitmap(tile, x, y, null);
+                c.drawBitmap(tile, x - mRenderOffset.x, y - mRenderOffset.y, null);
                 tile.recycle();
 
                 // Progress the buffer to the next tile
-                tileBuffer.position(tileSize.getArea() * bpp);
+                tileBuffer.position(TILE_SIZE.getArea() * bpp);
                 tileBuffer = tileBuffer.slice();
             }
         }
     }
 
     public Bitmap getBitmap() {
+        if (mTileLayer == null)
+            return null;
+
         // Begin a tile transaction, otherwise the buffer can be destroyed while
         // we're reading from it.
         beginTransaction(mTileLayer);
@@ -311,12 +358,8 @@ public class GeckoSoftwareLayerClient extends LayerClient implements GeckoEventL
 
                 if (mTileLayer instanceof MultiTileLayer) {
                     b = Bitmap.createBitmap(mBufferSize.width, mBufferSize.height,
-                                               CairoUtils.cairoFormatTobitmapConfig(mFormat));
+                                            CairoUtils.cairoFormatTobitmapConfig(mFormat));
                     copyPixelsFromMultiTileLayer(b);
-                } else if (mTileLayer instanceof SingleTileLayer) {
-                    b = Bitmap.createBitmap(mBufferSize.width, mBufferSize.height,
-                                               CairoUtils.cairoFormatTobitmapConfig(mFormat));
-                    b.copyPixelsFromBuffer(mBuffer.asIntBuffer());
                 } else {
                     Log.w(LOGTAG, "getBitmap() called on a layer (" + mTileLayer + ") we don't know how to get a bitmap from");
                 }
@@ -336,6 +379,10 @@ public class GeckoSoftwareLayerClient extends LayerClient implements GeckoEventL
         return mBuffer;
     }
 
+    public Point getRenderOffset() {
+        return mRenderOffset;
+    }
+
     /**
      * Gecko calls this function to signal that it is done with the back buffer. After this call,
      * it is forbidden for Gecko to touch the buffer.
@@ -370,7 +417,7 @@ public class GeckoSoftwareLayerClient extends LayerClient implements GeckoEventL
 
         // Round up depending on layer implementation to remove texture wastage
         if (mTileLayer instanceof MultiTileLayer) {
-            // Round to the next multiple of the tile size, respecting maximum texture size
+            // Round to the next multiple of the tile size
             bufferSize = new IntSize(((mScreenSize.width + LayerController.MIN_BUFFER.width - 1) / TILE_SIZE.width + 1) * TILE_SIZE.width,
                                      ((mScreenSize.height + LayerController.MIN_BUFFER.height - 1) / TILE_SIZE.height + 1) * TILE_SIZE.height);
 
@@ -381,7 +428,7 @@ public class GeckoSoftwareLayerClient extends LayerClient implements GeckoEventL
             if (mScreenSize.width > maxSize || mScreenSize.height > maxSize)
                 throw new RuntimeException("Screen size of " + mScreenSize + " larger than maximum texture size of " + maxSize);
 
-            // Round to next power of two until we have NPOT texture support
+            // Round to next power of two until we have NPOT texture support, respecting maximum texture size
             bufferSize = new IntSize(Math.min(maxSize, IntSize.nextPowerOfTwo(mScreenSize.width + LayerController.MIN_BUFFER.width)),
                                      Math.min(maxSize, IntSize.nextPowerOfTwo(mScreenSize.height + LayerController.MIN_BUFFER.height)));
         }
@@ -452,6 +499,16 @@ public class GeckoSoftwareLayerClient extends LayerClient implements GeckoEventL
             GeckoAppShell.sendEventToGecko(new GeckoEvent(GeckoEvent.DRAW, rect));
         } else if ("Viewport:UpdateLater".equals(event)) {
             mUpdateViewportOnEndDraw = true;
+        } else if (("Document:Shown".equals(event) ||
+                    "Tab:Selected".equals(event)) &&
+                   (mTileLayer instanceof MultiTileLayer)) {
+            beginTransaction(mTileLayer);
+            try {
+                ((MultiTileLayer)mTileLayer).invalidateTiles();
+                ((MultiTileLayer)mTileLayer).invalidateBuffer();
+            } finally {
+                endTransaction(mTileLayer);
+            }
         }
     }
 }

mobile/android/base/gfx/Layer.java

@@ -63,23 +63,31 @@ public abstract class Layer {
 
     /**
      * Updates the layer. This returns false if there is still work to be done
-     * after this update.
+     * after this update. If the layer is not already in a transaction, the
+     * lock will be acquired and a transaction will automatically begin and
+     * end around the update.
      */
     public final boolean update(GL10 gl, RenderContext context) {
+        boolean startTransaction = true;
         if (mTransactionLock.isHeldByCurrentThread()) {
-            throw new RuntimeException("draw() called while transaction lock held by this " +
-                                       "thread?!");
+            startTransaction = false;
         }
 
-        if (mTransactionLock.tryLock()) {
-            try {
-                return performUpdates(gl, context);
-            } finally {
+        // If we're not already in a transaction and we can't acquire the lock,
+        // bail out.
+        if (startTransaction && !mTransactionLock.tryLock()) {
+            return false;
+        }
+
+        mInTransaction = true;
+        try {
+            return performUpdates(gl, context);
+        } finally {
+            if (startTransaction) {
+                mInTransaction = false;
                 mTransactionLock.unlock();
             }
         }
-
-        return false;
     }
 
     /** Subclasses override this function to draw the layer. */
@@ -109,7 +117,6 @@ public abstract class Layer {
         mTransactionLock.lock();
         mView = aView;
         mInTransaction = true;
-        mNewResolution = mResolution;
     }
 
     public void beginTransaction() {

mobile/android/base/gfx/LayerRenderer.java

@@ -101,12 +101,30 @@ public class LayerRenderer implements GLSurfaceView.Renderer {
 
         CairoImage backgroundImage = new BufferedCairoImage(controller.getBackgroundPattern());
         mBackgroundLayer = new SingleTileLayer(true, backgroundImage);
+        mBackgroundLayer.beginTransaction(null);
+        try {
+            mBackgroundLayer.invalidate();
+        } finally {
+            mBackgroundLayer.endTransaction();
+        }
 
         CairoImage checkerboardImage = new BufferedCairoImage(controller.getCheckerboardPattern());
         mCheckerboardLayer = new SingleTileLayer(true, checkerboardImage);
+        mCheckerboardLayer.beginTransaction(null);
+        try {
+            mCheckerboardLayer.invalidate();
+        } finally {
+            mCheckerboardLayer.endTransaction();
+        }
 
         CairoImage shadowImage = new BufferedCairoImage(controller.getShadowPattern());
         mShadowLayer = new NinePatchTileLayer(shadowImage);
+        mShadowLayer.beginTransaction(null);
+        try {
+            mShadowLayer.invalidate();
+        } finally {
+            mShadowLayer.endTransaction();
+        }
 
         IntSize frameRateLayerSize = new IntSize(FRAME_RATE_METER_WIDTH, FRAME_RATE_METER_HEIGHT);
         mFrameRateLayer = TextLayer.create(frameRateLayerSize, "-- ms/--");

mobile/android/base/gfx/MultiTileLayer.java

@@ -37,15 +37,21 @@
 
 package org.mozilla.gecko.gfx;
 
+import org.mozilla.gecko.FloatUtils;
 import org.mozilla.gecko.gfx.CairoImage;
 import org.mozilla.gecko.gfx.IntSize;
 import org.mozilla.gecko.gfx.SingleTileLayer;
 import android.graphics.Point;
+import android.graphics.PointF;
 import android.graphics.Rect;
 import android.graphics.RectF;
+import android.graphics.Region;
 import android.util.Log;
+import java.lang.Long;
 import java.nio.ByteBuffer;
-import java.util.ArrayList;
+import java.util.HashMap;
+import java.util.LinkedList;
+import java.util.ListIterator;
 import javax.microedition.khronos.opengles.GL10;
 
 /**
@@ -57,9 +63,12 @@ public class MultiTileLayer extends Layer {
     private static final String LOGTAG = "GeckoMultiTileLayer";
 
     private final CairoImage mImage;
-    private IntSize mTileSize;
+    private final IntSize mTileSize;
     private IntSize mBufferSize;
-    private final ArrayList<SubTile> mTiles;
+    private Region mDirtyRegion;
+    private Region mValidRegion;
+    private final LinkedList<SubTile> mTiles;
+    private final HashMap<Long, SubTile> mPositionHash;
 
     public MultiTileLayer(CairoImage image, IntSize tileSize) {
         super();
@@ -67,27 +76,37 @@ public class MultiTileLayer extends Layer {
         mImage = image;
         mTileSize = tileSize;
         mBufferSize = new IntSize(0, 0);
-        mTiles = new ArrayList<SubTile>();
+        mDirtyRegion = new Region();
+        mValidRegion = new Region();
+        mTiles = new LinkedList<SubTile>();
+        mPositionHash = new HashMap<Long, SubTile>();
     }
 
+    /**
+     * Invalidates a sub-region of the layer. Data will be uploaded from the
+     * backing buffer over subsequent calls to update().
+     * This method is only valid inside a transaction.
+     */
     public void invalidate(Rect dirtyRect) {
-        if (!inTransaction())
+        if (!inTransaction()) {
             throw new RuntimeException("invalidate() is only valid inside a transaction");
-
-        for (SubTile layer : mTiles) {
-            IntSize tileSize = layer.getSize();
-            Rect tileRect = new Rect(layer.x, layer.y, layer.x + tileSize.width, layer.y + tileSize.height);
-
-            if (tileRect.intersect(dirtyRect)) {
-                tileRect.offset(-layer.x, -layer.y);
-                layer.invalidate(tileRect);
-            }
         }
+
+        mDirtyRegion.union(dirtyRect);
+        mValidRegion.union(dirtyRect);
     }
 
-    public void invalidate() {
-        for (SubTile layer : mTiles)
-            layer.invalidate();
+    /**
+     * Invalidates the backing buffer. Data will not be uploaded from an invalid
+     * backing buffer. This method is only valid inside a transaction.
+     */
+    public void invalidateBuffer() {
+        if (!inTransaction()) {
+            throw new RuntimeException("invalidateBuffer() is only valid inside a transaction");
+        }
+
+        mDirtyRegion.setEmpty();
+        mValidRegion.setEmpty();
     }
 
     @Override
@@ -95,61 +114,104 @@ public class MultiTileLayer extends Layer {
         return mImage.getSize();
     }
 
+    /**
+     * Makes sure there are enough tiles to accommodate the buffer image.
+     */
     private void validateTiles() {
         IntSize size = getSize();
 
         if (size.equals(mBufferSize))
             return;
 
-        // Regenerate tiles
-        mTiles.clear();
-        int offset = 0;
-        final int format = mImage.getFormat();
-        final ByteBuffer buffer = mImage.getBuffer().slice();
-        final int bpp = CairoUtils.bitsPerPixelForCairoFormat(format) / 8;
-        for (int y = 0; y < size.height; y += mTileSize.height) {
-            for (int x = 0; x < size.width; x += mTileSize.width) {
-                // Create a CairoImage implementation that returns a
-                // tile from the parent CairoImage. It's assumed that
-                // the tiles are stored in series.
-                final IntSize layerSize =
-                    new IntSize(Math.min(mTileSize.width, size.width - x),
-                                Math.min(mTileSize.height, size.height - y));
-                final int tileOffset = offset;
+        mBufferSize = size;
 
-                CairoImage subImage = new CairoImage() {
-                    @Override
-                    public ByteBuffer getBuffer() {
-                        // Create a ByteBuffer that shares the data of the original
-                        // buffer, but is positioned and limited so that only the
-                        // tile data is accessible.
-                        buffer.position(tileOffset);
-                        ByteBuffer tileBuffer = buffer.slice();
-                        tileBuffer.limit(layerSize.getArea() * bpp);
+        // Shrink/grow tile pool
+        int nTiles = (Math.round(size.width / (float)mTileSize.width) + 1) *
+                     (Math.round(size.height / (float)mTileSize.height) + 1);
+        if (mTiles.size() < nTiles) {
+            Log.i(LOGTAG, "Tile pool growing from " + mTiles.size() + " to " + nTiles);
 
-                        return tileBuffer;
-                    }
+            for (int i = 0; i < nTiles; i++) {
+                mTiles.add(new SubTile(new SubImage(mImage, mTileSize)));
+            }
+        } else if (mTiles.size() > nTiles) {
+            Log.i(LOGTAG, "Tile pool shrinking from " + mTiles.size() + " to " + nTiles);
 
-                    @Override
-                    public IntSize getSize() {
-                        return layerSize;
-                    }
-
-                    @Override
-                    public int getFormat() {
-                        return format;
-                    }
-                };
-
-                mTiles.add(new SubTile(subImage, x, y));
-                offset += layerSize.getArea() * bpp;
+            // Remove tiles from the beginning of the list, as these are
+            // least recently used tiles
+            for (int i = mTiles.size(); i > nTiles; i--) {
+                SubTile tile = mTiles.get(0);
+                if (tile.key != null) {
+                    mPositionHash.remove(tile.key);
+                }
+                mTiles.remove(0);
             }
         }
 
-        // Set tile origins and resolution
-        refreshTileMetrics(getOrigin(), getResolution(), false);
+        // A buffer size probably means a layout change, so invalidate all tiles.
+        invalidateTiles();
+    }
 
-        mBufferSize = size;
+    /**
+     * Returns a Long representing the given Point. Used for hashing.
+     */
+    private Long longFromPoint(Point point) {
+        // Assign 32 bits for each dimension of the point.
+        return new Long((((long)point.x) << 32) | point.y);
+    }
+
+    /**
+     * Performs the necessary functions to update the specified properties of
+     * a sub-tile.
+     */
+    private void updateTile(GL10 gl, RenderContext context, SubTile tile, Point tileOrigin, Rect dirtyRect, boolean reused) {
+        tile.beginTransaction(null);
+        try {
+            if (reused) {
+                // Invalidate any area that isn't represented in the current
+                // buffer. This is done as SingleTileLayer always updates the
+                // entire width, regardless of the dirty-rect's width, and so
+                // can override existing data.
+                Point origin = getOrigin();
+                Rect validRect = tile.getValidTextureArea();
+                validRect.offset(tileOrigin.x - origin.x, tileOrigin.y - origin.y);
+                Region validRegion = new Region(validRect);
+                validRegion.op(mValidRegion, Region.Op.INTERSECT);
+
+                // SingleTileLayer can't draw complex regions, so in that case,
+                // just invalidate the entire area.
+                tile.invalidateTexture();
+                if (!validRegion.isComplex()) {
+                    validRect.set(validRegion.getBounds());
+                    validRect.offset(origin.x - tileOrigin.x, origin.y - tileOrigin.y);
+                }
+            } else {
+                // Update tile metrics
+                tile.setOrigin(tileOrigin);
+                tile.setResolution(getResolution());
+
+                // Make sure that non-reused tiles are marked as invalid before
+                // uploading new content.
+                tile.invalidateTexture();
+
+                // (Re)Place in the position hash for quick retrieval.
+                if (tile.key != null) {
+                    mPositionHash.remove(tile.key);
+                }
+                tile.key = longFromPoint(tileOrigin);
+                mPositionHash.put(tile.key, tile);
+            }
+
+            // Invalidate the area we want to upload.
+            tile.invalidate(dirtyRect);
+
+            // Perform updates and mark texture as valid.
+            if (!tile.performUpdates(gl, context)) {
+                Log.e(LOGTAG, "Sub-tile failed to update fully");
+            }
+        } finally {
+            tile.endTransaction();
+        }
     }
 
     @Override
@@ -158,96 +220,167 @@ public class MultiTileLayer extends Layer {
 
         validateTiles();
 
-        // Iterate over the tiles and decide which ones we'll be drawing
-        int dirtyTiles = 0;
-        boolean screenUpdateDone = false;
-        SubTile firstDirtyTile = null;
-        for (SubTile layer : mTiles) {
-            // First do a non-texture update to make sure coordinates are
-            // up-to-date.
-            boolean invalid = layer.getSkipTextureUpdate();
-            layer.setSkipTextureUpdate(true);
-            layer.performUpdates(gl, context);
+        // Bail out early if we have nothing to do.
+        if (mDirtyRegion.isEmpty() || mTiles.isEmpty()) {
+            return true;
+        }
 
-            RectF layerBounds = layer.getBounds(context, new FloatSize(layer.getSize()));
-            boolean isDirty = layer.isDirty();
+        // Check that we're capable of updating from this origin.
+        Point origin = getOrigin();
+        if ((origin.x % mTileSize.width) != 0 || (origin.y % mTileSize.height) != 0) {
+            Log.e(LOGTAG, "MultiTileLayer doesn't support non tile-aligned origins! (" +
+                  origin.x + ", " + origin.y + ")");
+            return true;
+        }
 
-            if (isDirty) {
-                if (!RectF.intersects(layerBounds, context.viewport)) {
-                    if (firstDirtyTile == null)
-                        firstDirtyTile = layer;
-                    dirtyTiles ++;
-                    invalid = true;
-                } else {
-                    // This tile intersects with the screen and is dirty,
-                    // update it immediately.
-                    layer.setSkipTextureUpdate(false);
-                    screenUpdateDone = true;
-                    layer.performUpdates(gl, context);
-                    invalid = false;
-                }
+        // Transform the viewport into tile-space so we can see what part of the
+        // dirty region intersects with it.
+        // We update any tiles intersecting with the screen before tiles
+        // intersecting with the viewport.
+        // XXX Maybe we want to to split this update even further to update
+        //     checkerboard area before updating screen regions with old data.
+        //     Note that this could provide inconsistent views, so we may not
+        //     want to do this.
+        Rect tilespaceViewport;
+        float scaleFactor = getResolution() / context.zoomFactor;
+        tilespaceViewport = RectUtils.roundOut(RectUtils.scale(context.viewport, scaleFactor));
+        tilespaceViewport.offset(-origin.x, -origin.y);
+
+        // Expand tile-space viewport to tile boundaries
+        tilespaceViewport.left = (tilespaceViewport.left / mTileSize.width) * mTileSize.width;
+        tilespaceViewport.right += mTileSize.width - 1;
+        tilespaceViewport.right = (tilespaceViewport.right / mTileSize.width) * mTileSize.width;
+        tilespaceViewport.top = (tilespaceViewport.top / mTileSize.height) * mTileSize.height;
+        tilespaceViewport.bottom += mTileSize.height - 1;
+        tilespaceViewport.bottom = (tilespaceViewport.bottom / mTileSize.height) * mTileSize.height;
+
+        // Declare a region for storing the results of Region operations
+        Region opRegion = new Region();
+
+        // Test if the dirty region intersects with the screen
+        boolean updateVisible = false;
+        Region updateRegion = mDirtyRegion;
+        if (opRegion.op(tilespaceViewport, mDirtyRegion, Region.Op.INTERSECT)) {
+            updateVisible = true;
+            updateRegion = new Region(opRegion);
+        }
+
+        // Invalidate any tiles that are due to be replaced if their resolution
+        // doesn't match the parent layer resolution, and any tiles that are
+        // off-screen and off-buffer, as we cannot guarantee their validity.
+        //
+        // Note that we also cannot guarantee the validity of on-screen,
+        // off-buffer tiles, but this is a rare case that we allow for
+        // optimisation purposes.
+        //
+        // XXX Ideally, we want to remove this second invalidation clause
+        //     somehow. It may be possible to know if off-screen tiles are
+        //     valid by monitoring reflows on the browser element, or
+        //     something along these lines.
+        LinkedList<SubTile> invalidTiles = new LinkedList<SubTile>();
+        Rect bounds = mValidRegion.getBounds();
+        for (ListIterator<SubTile> i = mTiles.listIterator(); i.hasNext();) {
+            SubTile tile = i.next();
+
+            if (tile.key == null) {
+                continue;
             }
 
-            // We use the SkipTextureUpdate flag as a marker of a tile's
-            // validity. This is required, as sometimes layers are drawn
-            // without updating first, and we mustn't draw tiles that have
-            // been marked as invalid that we haven't updated.
-            layer.setSkipTextureUpdate(invalid);
+            RectF tileBounds = tile.getBounds(context, new FloatSize(tile.getSize()));
+            Rect tilespaceTileBounds =
+                RectUtils.round(RectUtils.scale(tileBounds, scaleFactor));
+            tilespaceTileBounds.offset(-origin.x, -origin.y);
+
+            // First bracketed clause: Invalidate off-screen, off-buffer tiles
+            // Second: Invalidate visible tiles at the wrong resolution that have updates
+            if ((!Rect.intersects(bounds, tilespaceTileBounds) &&
+                 !Rect.intersects(tilespaceViewport, tilespaceTileBounds)) ||
+                (!FloatUtils.fuzzyEquals(tile.getResolution(), getResolution()) &&
+                 opRegion.op(tilespaceTileBounds, updateRegion, Region.Op.INTERSECT))) {
+                tile.invalidateTexture();
+
+                // Add to the list of invalid tiles and remove from the main list
+                invalidTiles.add(tile);
+                i.remove();
+
+                // Remove from the position hash
+                mPositionHash.remove(tile.key);
+                tile.key = null;
+            }
         }
 
-        // Now if no tiles that intersect with the screen were updated, update
-        // a single tile that doesn't (if there are any). This has the effect
-        // of spreading out non-critical texture upload over time, and smoothing
-        // upload-related hitches.
-        if (!screenUpdateDone && firstDirtyTile != null) {
-            firstDirtyTile.setSkipTextureUpdate(false);
-            firstDirtyTile.performUpdates(gl, context);
-            dirtyTiles --;
+        // Push invalid tiles to the head of the queue so they get used first
+        mTiles.addAll(0, invalidTiles);
+
+        // Update tiles
+        // Note, it's <= as the buffer is over-allocated due to render-offsetting.
+        for (int y = origin.y; y <= origin.y + mBufferSize.height; y += mTileSize.height) {
+            for (int x = origin.x; x <= origin.x + mBufferSize.width; x += mTileSize.width) {
+                // Does this tile intersect with the dirty region?
+                Rect tilespaceTileRect = new Rect(x - origin.x, y - origin.y,
+                                                  (x - origin.x) + mTileSize.width,
+                                                  (y - origin.y) + mTileSize.height);
+                if (!opRegion.op(tilespaceTileRect, updateRegion, Region.Op.INTERSECT)) {
+                    continue;
+                }
+
+                // Dirty tile, find out if we already have this tile to reuse.
+                boolean reusedTile = true;
+                Point tileOrigin = new Point(x, y);
+                SubTile tile = mPositionHash.get(longFromPoint(tileOrigin));
+
+                // If we don't, get an unused tile (we store these at the head of the list).
+                if (tile == null) {
+                    tile = mTiles.removeFirst();
+                    reusedTile = false;
+                } else {
+                    mTiles.remove(tile);
+                }
+
+                // Place tile at the end of the tile-list so it isn't re-used.
+                mTiles.add(tile);
+
+                // Work out the tile's invalid area in this tile's space.
+                if (opRegion.isComplex()) {
+                    Log.w(LOGTAG, "MultiTileLayer encountered complex dirty region");
+                }
+                Rect dirtyRect = opRegion.getBounds();
+                dirtyRect.offset(origin.x - x, origin.y - y);
+
+                // Update tile metrics and texture data
+                tile.x = (x - origin.x) / mTileSize.width;
+                tile.y = (y - origin.y) / mTileSize.height;
+                updateTile(gl, context, tile, tileOrigin, dirtyRect, reusedTile);
+
+                // If this update isn't visible, we only want to update one
+                // tile at a time.
+                if (!updateVisible) {
+                    mDirtyRegion.op(opRegion, Region.Op.XOR);
+                    return mDirtyRegion.isEmpty();
+                }
+            }
         }
 
-        return (dirtyTiles == 0);
-    }
+        // Remove the update region from the dirty region
+        mDirtyRegion.op(updateRegion, Region.Op.XOR);
 
-    private void refreshTileMetrics(Point origin, float resolution, boolean inTransaction) {
-        IntSize size = getSize();
-        for (SubTile layer : mTiles) {
-            if (!inTransaction)
-                layer.beginTransaction(null);
-
-            if (origin != null)
-                layer.setOrigin(new Point(origin.x + layer.x, origin.y + layer.y));
-            if (resolution >= 0.0f)
-                layer.setResolution(resolution);
-
-            if (!inTransaction)
-                layer.endTransaction();
-        }
-    }
-
-    @Override
-    public void setOrigin(Point newOrigin) {
-        super.setOrigin(newOrigin);
-        refreshTileMetrics(newOrigin, -1, true);
-    }
-
-    @Override
-    public void setResolution(float newResolution) {
-        super.setResolution(newResolution);
-        refreshTileMetrics(null, newResolution, true);
+        return mDirtyRegion.isEmpty();
     }
 
     @Override
     public void beginTransaction(LayerView aView) {
         super.beginTransaction(aView);
 
-        for (SubTile layer : mTiles)
+        for (SubTile layer : mTiles) {
             layer.beginTransaction(aView);
+        }
     }
 
     @Override
     public void endTransaction() {
-        for (SubTile layer : mTiles)
+        for (SubTile layer : mTiles) {
             layer.endTransaction();
+        }
 
         super.endTransaction();
     }
@@ -255,11 +388,6 @@ public class MultiTileLayer extends Layer {
     @Override
     public void draw(RenderContext context) {
         for (SubTile layer : mTiles) {
-            // We use the SkipTextureUpdate flag as a validity flag. If it's false,
-            // the contents of this tile are invalid and we shouldn't draw it.
-            if (layer.getSkipTextureUpdate())
-                continue;
-
             // Avoid work, only draw tiles that intersect with the viewport
             RectF layerBounds = layer.getBounds(context, new FloatSize(layer.getSize()));
             if (RectF.intersects(layerBounds, context.viewport))
@@ -267,14 +395,90 @@ public class MultiTileLayer extends Layer {
         }
     }
 
-    class SubTile extends SingleTileLayer {
+    /**
+     * Invalidates all sub-tiles. This should be called if the source backing
+     * this layer has changed. This method is only valid inside a transaction.
+     */
+    public void invalidateTiles() {
+        if (!inTransaction()) {
+            throw new RuntimeException("invalidateTiles() is only valid inside a transaction");
+        }
+
+        for (SubTile tile : mTiles) {
+            // Remove tile from position hash and mark it as invalid
+            if (tile.key != null) {
+                mPositionHash.remove(tile.key);
+                tile.key = null;
+            }
+            tile.invalidateTexture();
+        }
+    }
+
+    /**
+     * A SingleTileLayer extension with fields for relevant tile data that
+     * MultiTileLayer requires.
+     */
+    private static class SubTile extends SingleTileLayer {
         public int x;
         public int y;
 
-        public SubTile(CairoImage mImage, int mX, int mY) {
-            super(mImage);
-            x = mX;
-            y = mY;
+        public Long key;
+
+        public SubTile(SubImage aImage) {
+            super(aImage);
+
+            aImage.tile = this;
+        }
+    }
+
+    /**
+     * A CairoImage implementation that returns a tile from a parent CairoImage.
+     * This assumes that the parent image has a size that is a multiple of the
+     * tile size.
+     */
+    private static class SubImage extends CairoImage {
+        public SubTile tile;
+
+        private IntSize mTileSize;
+        private CairoImage mImage;
+
+        public SubImage(CairoImage image, IntSize tileSize) {
+            mTileSize = tileSize;
+            mImage = image;
+        }
+
+        @Override
+        public ByteBuffer getBuffer() {
+            // Create a ByteBuffer that shares the data of the original
+            // buffer, but is positioned and limited so that only the
+            // tile data is accessible.
+            IntSize bufferSize = mImage.getSize();
+            int bpp = CairoUtils.bitsPerPixelForCairoFormat(getFormat()) / 8;
+            int index = (tile.y * (bufferSize.width / mTileSize.width + 1)) + tile.x;
+
+            ByteBuffer buffer = mImage.getBuffer().slice();
+
+            try {
+                buffer.position(index * mTileSize.getArea() * bpp);
+                buffer = buffer.slice();
+                buffer.limit(mTileSize.getArea() * bpp);
+            } catch (IllegalArgumentException e) {
+                Log.e(LOGTAG, "Tile image-data out of bounds! Tile: (" +
+                      tile.x + ", " + tile.y + "), image (" + bufferSize + ")");
+                return null;
+            }
+
+            return buffer;
+        }
+
+        @Override
+        public IntSize getSize() {
+            return mTileSize;
+        }
+
+        @Override
+        public int getFormat() {
+            return mImage.getFormat();
         }
     }
 }

mobile/android/base/gfx/RectUtils.java

@@ -99,11 +99,18 @@ public final class RectUtils {
                          y + (rect.height() * scale));
     }
 
+    /** Returns the nearest integer rect of the given rect. */
     public static Rect round(RectF rect) {
         return new Rect(Math.round(rect.left), Math.round(rect.top),
                         Math.round(rect.right), Math.round(rect.bottom));
     }
 
+    /** Returns the smallest integer rect that encapsulates the given rect. */
+    public static Rect roundOut(RectF rect) {
+        return new Rect((int)Math.floor(rect.left), (int)Math.floor(rect.top),
+                        (int)Math.ceil(rect.right), (int)Math.ceil(rect.bottom));
+    }
+
     public static IntSize getSize(Rect rect) {
         return new IntSize(rect.width(), rect.height());
     }

mobile/android/base/gfx/SingleTileLayer.java

@@ -43,6 +43,7 @@ import org.mozilla.gecko.gfx.IntSize;
 import org.mozilla.gecko.gfx.LayerController;
 import org.mozilla.gecko.gfx.TileLayer;
 import android.graphics.PointF;
+import android.graphics.Rect;
 import android.graphics.RectF;
 import android.opengl.GLES11;
 import android.opengl.GLES11Ext;
@@ -58,6 +59,8 @@ import javax.microedition.khronos.opengles.GL10;
  * TODO: Repeating textures really should be their own type of layer.
  */
 public class SingleTileLayer extends TileLayer {
+    private static final String LOGTAG = "GeckoSingleTileLayer";
+
     public SingleTileLayer(CairoImage image) { this(false, image); }
 
     public SingleTileLayer(boolean repeat, CairoImage image) {
@@ -71,6 +74,11 @@ public class SingleTileLayer extends TileLayer {
         if (!initialized())
             return;
 
+        // If the texture contents is entirely invalid, we have nothing to draw.
+        Rect validTexture = getValidTextureArea();
+        if (validTexture.isEmpty())
+            return;
+
         GLES11.glBindTexture(GL10.GL_TEXTURE_2D, getTextureID());
 
         RectF bounds;
@@ -79,13 +87,27 @@ public class SingleTileLayer extends TileLayer {
         RectF viewport = context.viewport;
 
         if (repeats()) {
+            if (!validTexture.equals(new Rect(0, 0, size.width, size.height))) {
+                Log.e(LOGTAG, "Drawing partial repeating textures is unsupported!");
+            }
+
             bounds = new RectF(0.0f, 0.0f, viewport.width(), viewport.height());
             int width = Math.round(viewport.width());
             int height = Math.round(-viewport.height());
             cropRect = new int[] { 0, size.height, width, height };
         } else {
             bounds = getBounds(context, new FloatSize(size));
-            cropRect = new int[] { 0, size.height, size.width, -size.height };
+
+            float scaleFactor = bounds.width() / (float)size.width;
+            bounds.left += validTexture.left * scaleFactor;
+            bounds.top += validTexture.top * scaleFactor;
+            bounds.right -= (size.width - validTexture.right) * scaleFactor;
+            bounds.bottom -= (size.height - validTexture.bottom) * scaleFactor;
+
+            cropRect = new int[] { validTexture.left,
+                                   validTexture.bottom,
+                                   validTexture.width(),
+                                   -validTexture.height() };
         }
 
         GLES11.glTexParameteriv(GL10.GL_TEXTURE_2D, GLES11Ext.GL_TEXTURE_CROP_RECT_OES, cropRect,

mobile/android/base/gfx/TileLayer.java

@@ -59,14 +59,14 @@ public abstract class TileLayer extends Layer {
     private final CairoImage mImage;
     private final boolean mRepeat;
     private IntSize mSize;
-    private boolean mSkipTextureUpdate;
+    private Rect mValidTextureRect;
     private int[] mTextureIDs;
 
     public TileLayer(boolean repeat, CairoImage image) {
         mRepeat = repeat;
         mImage = image;
         mSize = new IntSize(0, 0);
-        mSkipTextureUpdate = false;
+        mValidTextureRect = new Rect();
 
         IntSize bufferSize = mImage.getSize();
         mDirtyRect = new Rect();
@@ -132,22 +132,27 @@ public abstract class TileLayer extends Layer {
         }
     }
 
-    /** Tells the tile not to update the texture on the next update. */
-    public void setSkipTextureUpdate(boolean skip) {
-        mSkipTextureUpdate = skip;
+    /**
+     * Tells the tile that its texture contents are invalid. This will also
+     * clear any invalidated area.
+     */
+    public void invalidateTexture() {
+        mValidTextureRect.setEmpty();
+        mDirtyRect.setEmpty();
     }
 
-    public boolean getSkipTextureUpdate() {
-        return mSkipTextureUpdate;
+    /**
+     * Returns a handle to the valid texture area rectangle. Modifying this
+     * Rect will modify the valid texture area for this layer.
+     */
+    public Rect getValidTextureArea() {
+        return mValidTextureRect;
     }
 
     @Override
     protected boolean performUpdates(GL10 gl, RenderContext context) {
         super.performUpdates(gl, context);
 
-        if (mSkipTextureUpdate)
-            return false;
-
         // Reallocate the texture if the size has changed
         validateTexture(gl);
 
@@ -155,23 +160,20 @@ public abstract class TileLayer extends Layer {
         if (!mImage.getSize().isPositive())
             return true;
 
-        // If we haven't allocated a texture, assume the whole region is dirty
-        if (mTextureIDs == null)
-            uploadFullTexture(gl);
-        else
-            uploadDirtyRect(gl, mDirtyRect);
-
+        // Update the dirty region
+        uploadDirtyRect(gl, mDirtyRect);
         mDirtyRect.setEmpty();
 
         return true;
     }
 
-    private void uploadFullTexture(GL10 gl) {
-        IntSize bufferSize = mImage.getSize();
-        uploadDirtyRect(gl, new Rect(0, 0, bufferSize.width, bufferSize.height));
-    }
-
     private void uploadDirtyRect(GL10 gl, Rect dirtyRect) {
+        IntSize bufferSize = mImage.getSize();
+        Rect bufferRect = new Rect(0, 0, bufferSize.width, bufferSize.height);
+
+        // Make sure the dirty region intersects with the buffer
+        dirtyRect.intersect(bufferRect);
+
         // If we have nothing to upload, just return for now
         if (dirtyRect.isEmpty())
             return;
@@ -181,6 +183,11 @@ public abstract class TileLayer extends Layer {
         if (imageBuffer == null)
             return;
 
+        // Mark the dirty region as valid. Note, we assume that the valid area
+        // can be enclosed by a rectangle (ideally we'd use a Region, but it'd
+        // be slower and it probably isn't necessary).
+        mValidTextureRect.union(dirtyRect);
+
         boolean newlyCreated = false;
 
         if (mTextureIDs == null) {
@@ -189,15 +196,12 @@ public abstract class TileLayer extends Layer {
             newlyCreated = true;
         }
 
-        IntSize bufferSize = mImage.getSize();
-        Rect bufferRect = new Rect(0, 0, bufferSize.width, bufferSize.height);
-
         int cairoFormat = mImage.getFormat();
         CairoGLInfo glInfo = new CairoGLInfo(cairoFormat);
 
         bindAndSetGLParameters(gl);
 
-        if (newlyCreated || dirtyRect.contains(bufferRect)) {
+        if (newlyCreated || dirtyRect.equals(bufferRect)) {
             if (mSize.equals(bufferSize)) {
                 gl.glTexImage2D(gl.GL_TEXTURE_2D, 0, glInfo.internalFormat, mSize.width, mSize.height,
                                 0, glInfo.format, glInfo.type, imageBuffer);
@@ -211,11 +215,6 @@ public abstract class TileLayer extends Layer {
             }
         }
 
-        // Make sure that the dirty region intersects with the buffer rect,
-        // otherwise we'll end up with an invalid buffer pointer.
-        if (!Rect.intersects(dirtyRect, bufferRect))
-            return;
-
         /*
          * Upload the changed rect. We have to widen to the full width of the texture
          * because we can't count on the device having support for GL_EXT_unpack_subimage,
@@ -232,8 +231,8 @@ public abstract class TileLayer extends Layer {
         }
 
         viewBuffer.position(position);
-        gl.glTexSubImage2D(gl.GL_TEXTURE_2D, 0, 0, dirtyRect.top, bufferSize.width,
-                           Math.min(bufferSize.height - dirtyRect.top, dirtyRect.height()),
+        gl.glTexSubImage2D(gl.GL_TEXTURE_2D, 0, 0, dirtyRect.top,
+                           bufferSize.width, dirtyRect.height(),
                            glInfo.format, glInfo.type, viewBuffer);
     }
 

mobile/android/chrome/content/browser.js

@@ -2023,6 +2023,7 @@ Tab.prototype = {
         // Is it on the top level?
         let contentDocument = aSubject;
         if (contentDocument == this.browser.contentDocument) {
+          sendMessageToJava({ gecko: { type: "Document:Shown" } });
           ViewportHandler.updateMetadata(this);
           this.documentIdForCurrentViewport = ViewportHandler.getIdForDocument(contentDocument);
         }

widget/android/AndroidJavaWrappers.cpp

@@ -107,6 +107,7 @@ jmethodID AndroidAddress::jGetThoroughfareMethod;
 jclass AndroidGeckoSoftwareLayerClient::jGeckoSoftwareLayerClientClass = 0;
 jmethodID AndroidGeckoSoftwareLayerClient::jLockBufferMethod = 0;
 jmethodID AndroidGeckoSoftwareLayerClient::jUnlockBufferMethod = 0;
+jmethodID AndroidGeckoSoftwareLayerClient::jGetRenderOffsetMethod = 0;
 jmethodID AndroidGeckoSoftwareLayerClient::jBeginDrawingMethod = 0;
 jmethodID AndroidGeckoSoftwareLayerClient::jEndDrawingMethod = 0;
 jclass AndroidGeckoSurfaceView::jGeckoSurfaceViewClass = 0;
@@ -323,8 +324,9 @@ AndroidGeckoSoftwareLayerClient::InitGeckoSoftwareLayerClientClass(JNIEnv *jEnv)
 
     jLockBufferMethod = getMethod("lockBuffer", "()Ljava/nio/ByteBuffer;");
     jUnlockBufferMethod = getMethod("unlockBuffer", "()V");
-    jBeginDrawingMethod = getMethod("beginDrawing", "(II)V");
-    jEndDrawingMethod = getMethod("endDrawing", "(IIIILjava/lang/String;Z)V");
+    jGetRenderOffsetMethod = getMethod("getRenderOffset", "()Landroid/graphics/Point;");
+    jBeginDrawingMethod = getMethod("beginDrawing", "(IILjava/lang/String;Z)Z");
+    jEndDrawingMethod = getMethod("endDrawing", "(IIII)V");
 #endif
 }
 
@@ -618,21 +620,28 @@ AndroidGeckoSoftwareLayerClient::UnlockBuffer()
 }
 
 void
-AndroidGeckoSoftwareLayerClient::BeginDrawing(int aWidth, int aHeight)
+AndroidGeckoSoftwareLayerClient::GetRenderOffset(nsIntPoint &aOffset)
+{
+    AndroidPoint offset(JNI(), JNI()->CallObjectMethod(wrapped_obj, jGetRenderOffsetMethod));
+    aOffset.x = offset.X();
+    aOffset.y = offset.Y();
+}
+
+bool
+AndroidGeckoSoftwareLayerClient::BeginDrawing(int aWidth, int aHeight, const nsAString &aMetadata, bool aHasDirectTexture)
 {
     NS_ASSERTION(!isNull(), "BeginDrawing() called on null software layer client!");
     AndroidBridge::AutoLocalJNIFrame(1);
-    return JNI()->CallVoidMethod(wrapped_obj, jBeginDrawingMethod, aWidth, aHeight);
+    jstring jMetadata = JNI()->NewString(nsPromiseFlatString(aMetadata).get(), aMetadata.Length());
+    return JNI()->CallBooleanMethod(wrapped_obj, jBeginDrawingMethod, aWidth, aHeight, jMetadata, aHasDirectTexture);
 }
 
 void
-AndroidGeckoSoftwareLayerClient::EndDrawing(const nsIntRect &aRect, const nsAString &aMetadata, bool aHasDirectTexture)
+AndroidGeckoSoftwareLayerClient::EndDrawing(const nsIntRect &aRect)
 {
     NS_ASSERTION(!isNull(), "EndDrawing() called on null software layer client!");
     AndroidBridge::AutoLocalJNIFrame(1);
-    jstring jMetadata = JNI()->NewString(nsPromiseFlatString(aMetadata).get(), aMetadata.Length());
-    return JNI()->CallVoidMethod(wrapped_obj, jEndDrawingMethod, aRect.x, aRect.y, aRect.width,
-                                 aRect.height, jMetadata, aHasDirectTexture);
+    return JNI()->CallVoidMethod(wrapped_obj, jEndDrawingMethod, aRect.x, aRect.y, aRect.width, aRect.height);
 }
 
 jobject

widget/android/AndroidJavaWrappers.h

@@ -161,8 +161,9 @@ public:
     jobject LockBuffer();
     unsigned char *LockBufferBits();
     void UnlockBuffer();
-    void BeginDrawing(int aWidth, int aHeight);
-    void EndDrawing(const nsIntRect &aRect, const nsAString &aMetadata, bool aHasDirectTexture);
+    void GetRenderOffset(nsIntPoint &aOffset);
+    bool BeginDrawing(int aWidth, int aHeight, const nsAString &aMetadata, bool aHasDirectTexture);
+    void EndDrawing(const nsIntRect &aRect);
 
 private:
     static jclass jGeckoSoftwareLayerClientClass;
@@ -170,6 +171,7 @@ private:
     static jmethodID jUnlockBufferMethod;
 
 protected:
+     static jmethodID jGetRenderOffsetMethod;
      static jmethodID jBeginDrawingMethod;
      static jmethodID jEndDrawingMethod;
 };

widget/android/nsWindow.cpp

@@ -1195,13 +1195,22 @@ nsWindow::OnDraw(AndroidGeckoEvent *ae)
         return;
     }
 
+    nsAutoString metadata;
+    if (metadataProvider) {
+        metadataProvider->GetDrawMetadata(metadata);
+    }
+
     AndroidGeckoSoftwareLayerClient &client =
         AndroidBridge::Bridge()->GetSoftwareLayerClient();
-    client.BeginDrawing(gAndroidBounds.width, gAndroidBounds.height);
+    if (!client.BeginDrawing(gAndroidBounds.width, gAndroidBounds.height, metadata, HasDirectTexture())) {
+        return;
+    }
+
+    nsIntPoint renderOffset;
+    client.GetRenderOffset(renderOffset);
 
     nsIntRect dirtyRect = ae->Rect().Intersect(nsIntRect(0, 0, gAndroidBounds.width, gAndroidBounds.height));
 
-    nsAutoString metadata;
     unsigned char *bits = NULL;
     if (HasDirectTexture()) {
       if (sDirectTexture->Width() != gAndroidBounds.width ||
@@ -1220,37 +1229,32 @@ nsWindow::OnDraw(AndroidGeckoEvent *ae)
         int tileWidth = (gAndroidTileSize.width > 0) ? gAndroidTileSize.width : gAndroidBounds.width;
         int tileHeight = (gAndroidTileSize.height > 0) ? gAndroidTileSize.height : gAndroidBounds.height;
 
-        bool drawSuccess = true;
         int offset = 0;
 
-        for (int y = 0; y < gAndroidBounds.height; y += tileHeight) {
-            for (int x = 0; x < gAndroidBounds.width; x += tileWidth) {
-                int width = NS_MIN(tileWidth, gAndroidBounds.width - x);
-                int height = NS_MIN(tileHeight, gAndroidBounds.height - y);
+        // It is assumed that the buffer has been over-allocated so that not
+        // only is the tile-size constant, but that a render-offset of anything
+        // up to (but not including) the tile size could be accommodated.
+        for (int y = 0; y < gAndroidBounds.height + gAndroidTileSize.height; y += tileHeight) {
+            for (int x = 0; x < gAndroidBounds.width + gAndroidTileSize.width; x += tileWidth) {
 
                 nsRefPtr<gfxImageSurface> targetSurface =
                     new gfxImageSurface(bits + offset,
-                                        gfxIntSize(width, height),
-                                        width * 2,
+                                        gfxIntSize(tileWidth, tileHeight),
+                                        tileWidth * 2,
                                         gfxASurface::ImageFormatRGB16_565);
 
-                offset += width * height * 2;
+                offset += tileWidth * tileHeight * 2;
 
                 if (targetSurface->CairoStatus()) {
                     ALOG("### Failed to create a valid surface from the bitmap");
-                    drawSuccess = false;
                     break;
                 } else {
-                    targetSurface->SetDeviceOffset(gfxPoint(-x, -y));
+                    targetSurface->SetDeviceOffset(gfxPoint(renderOffset.x - x,
+                                                            renderOffset.y - y));
                     DrawTo(targetSurface, dirtyRect);
                 }
             }
         }
-
-        // Don't fill in the draw metadata on an unsuccessful draw
-        if (drawSuccess && metadataProvider) {
-            metadataProvider->GetDrawMetadata(metadata);
-        }
     }
 
     if (HasDirectTexture()) {
@@ -1259,7 +1263,7 @@ nsWindow::OnDraw(AndroidGeckoEvent *ae)
         client.UnlockBuffer();
     }
 
-    client.EndDrawing(dirtyRect, metadata, HasDirectTexture());
+    client.EndDrawing(dirtyRect);
     return;
 #endif