Bug 1024126 - Fix rounding errors and add a danger zone to the B2G AboutToCheckerboard heuristic function. r=Cwiiis

gfx/layers/apz/src/AsyncPanZoomController.cpp

@@ -160,6 +160,20 @@ typedef GeckoContentController::APZStateChange APZStateChange;
  * "apz.cross_slide_enabled"
  * Pref that enables integration with the Metro "cross-slide" gesture.
  *
+ * "apz.danger_zone_x"
+ * "apz.danger_zone_y"
+ * When drawing high-res tiles, we drop down to drawing low-res tiles
+ * when we know we can't keep up with the scrolling. The way we determine
+ * this is by checking if we are entering the "danger zone", which is the
+ * boundary of the painted content. For example, if the painted content
+ * goes from y=0...1000 and the visible portion is y=250...750 then
+ * we're far from checkerboarding. If we get to y=490...990 though then we're
+ * only 10 pixels away from showing checkerboarding so we are probably in
+ * a state where we can't keep up with scrolling. The danger zone prefs specify
+ * how wide this margin is; in the above example a y-axis danger zone of 10
+ * pixels would make us drop to low-res at y=490...990.
+ * This value is in layer pixels.
+ *
  * "apz.enlarge_displayport_when_clipped"
  * Pref that enables enlarging of the displayport along one axis when the
  * generated displayport's size is beyond that of the scrollable rect on the

gfx/layers/client/TiledContentClient.cpp

@@ -249,13 +249,29 @@ bool
 SharedFrameMetricsHelper::AboutToCheckerboard(const FrameMetrics& aContentMetrics,
                                               const FrameMetrics& aCompositorMetrics)
 {
-  CSSRect painted =
-        (aContentMetrics.mCriticalDisplayPort.IsEmpty() ? aContentMetrics.mDisplayPort : aContentMetrics.mCriticalDisplayPort)
-        + aContentMetrics.GetScrollOffset();
-  // Inflate painted by some small epsilon to deal with rounding
-  // error. We should replace this with a FuzzyContains function.
-  painted.Inflate(0.01f);
-  CSSRect showing = CSSRect(aCompositorMetrics.GetScrollOffset(), aCompositorMetrics.CalculateBoundedCompositedSizeInCssPixels());
+  // The size of the painted area is originally computed in layer pixels in layout, but then
+  // converted to app units and then back to CSS pixels before being put in the FrameMetrics.
+  // This process can introduce some rounding error, so we inflate the rect by one app unit
+  // to account for that.
+  CSSRect painted = (aContentMetrics.mCriticalDisplayPort.IsEmpty()
+                      ? aContentMetrics.mDisplayPort
+                      : aContentMetrics.mCriticalDisplayPort)
+                    + aContentMetrics.GetScrollOffset();
+  painted.Inflate(CSSMargin::FromAppUnits(nsMargin(1, 1, 1, 1)));
+
+  // Inflate the rect by the danger zone. See the description of the danger zone prefs
+  // in AsyncPanZoomController.cpp for an explanation of this.
+  CSSRect showing = CSSRect(aCompositorMetrics.GetScrollOffset(),
+                            aCompositorMetrics.CalculateBoundedCompositedSizeInCssPixels());
+  showing.Inflate(LayerSize(gfxPrefs::APZDangerZoneX(), gfxPrefs::APZDangerZoneY())
+                  / aCompositorMetrics.LayersPixelsPerCSSPixel());
+
+  // Clamp both rects to the scrollable rect, because having either of those
+  // exceed the scrollable rect doesn't make sense, and could lead to false
+  // positives.
+  painted = painted.Intersect(aContentMetrics.mScrollableRect);
+  showing = showing.Intersect(aContentMetrics.mScrollableRect);
+
   return !painted.Contains(showing);
 }
 

gfx/thebes/gfxPrefs.h

@@ -110,6 +110,8 @@ private:
   DECL_GFX_PREF(Live, "apz.axis_lock_mode",                    APZAxisLockMode, int32_t, 0);
   DECL_GFX_PREF(Live, "apz.content_response_timeout",          APZContentResponseTimeout, int32_t, 300);
   DECL_GFX_PREF(Live, "apz.cross_slide.enabled",               APZCrossSlideEnabled, bool, false);
+  DECL_GFX_PREF(Live, "apz.danger_zone_x",                     APZDangerZoneX, int32_t, 50);
+  DECL_GFX_PREF(Live, "apz.danger_zone_y",                     APZDangerZoneY, int32_t, 100);
   DECL_GFX_PREF(Live, "apz.enlarge_displayport_when_clipped",  APZEnlargeDisplayPortWhenClipped, bool, false);
   DECL_GFX_PREF(Live, "apz.fling_accel_interval_ms",           APZFlingAccelInterval, int32_t, 500);
   DECL_GFX_PREF(Live, "apz.fling_accel_base_mult",             APZFlingAccelBaseMultiplier, float, 1.0f);