Bug 1228597 - Clean up code paths that (un)apply a pres shell resolution. r=tn,botond

A clear separation is introduced between paths that deal with a root
document resolution (at the process boundary in e10s setups) and paths
that deal with a non-root document resolution (elsewhere in Layout code).

This allows both code paths to run on all platforms.

dom/base/nsContentUtils.cpp

@@ -7672,7 +7672,8 @@ nsContentUtils::ToWidgetPoint(const CSSPoint& aPoint,
                               nsPresContext* aPresContext)
 {
   return LayoutDeviceIntPoint::FromAppUnitsRounded(
-    (CSSPoint::ToAppUnits(aPoint) + aOffset).ApplyResolution(aPresContext->PresShell()->GetCumulativeScaleResolution()),
+    (CSSPoint::ToAppUnits(aPoint) +
+    aOffset).ApplyResolution(aPresContext->PresShell()->GetCumulativeNonRootScaleResolution()),
     aPresContext->AppUnitsPerDevPixel());
 }
 

dom/events/Event.cpp

@@ -936,7 +936,7 @@ Event::GetScreenCoords(nsPresContext* aPresContext,
     LayoutDevicePixel::ToAppUnits(aPoint, aPresContext->DeviceContext()->AppUnitsPerDevPixelAtUnitFullZoom());
 
   if (aPresContext->PresShell()) {
-    pt = pt.RemoveResolution(aPresContext->PresShell()->GetCumulativeScaleResolution());
+    pt = pt.RemoveResolution(aPresContext->PresShell()->GetCumulativeNonRootScaleResolution());
   }
 
   pt += LayoutDevicePixel::ToAppUnits(guiEvent->widget->WidgetToScreenOffset(),

gfx/layers/apz/util/APZCCallbackHelper.cpp

@@ -415,6 +415,28 @@ APZCCallbackHelper::GetRootContentDocumentPresShellForContent(nsIContent* aConte
     return context->PresShell();
 }
 
+static nsIPresShell*
+GetRootDocumentPresShell(nsIContent* aContent)
+{
+    nsIDocument* doc = aContent->GetComposedDoc();
+    if (!doc) {
+        return nullptr;
+    }
+    nsIPresShell* shell = doc->GetShell();
+    if (!shell) {
+        return nullptr;
+    }
+    nsPresContext* context = shell->GetPresContext();
+    if (!context) {
+        return nullptr;
+    }
+    context = context->GetRootPresContext();
+    if (!context) {
+        return nullptr;
+    }
+    return context->PresShell();
+}
+
 CSSPoint
 APZCCallbackHelper::ApplyCallbackTransform(const CSSPoint& aInput,
                                            const ScrollableLayerGuid& aGuid)
@@ -423,7 +445,21 @@ APZCCallbackHelper::ApplyCallbackTransform(const CSSPoint& aInput,
     if (aGuid.mScrollId == FrameMetrics::NULL_SCROLL_ID) {
         return input;
     }
-    // Apply the callback-transform.
+    nsCOMPtr<nsIContent> content = nsLayoutUtils::FindContentFor(aGuid.mScrollId);
+    if (!content) {
+        return input;
+    }
+
+    // First, scale inversely by the root content document's pres shell
+    // resolution to cancel the scale-to-resolution transform that the
+    // compositor adds to the layer with the pres shell resolution. The points
+    // sent to Gecko by APZ don't have this transform unapplied (unlike other
+    // compositor-side transforms) because APZ doesn't know about it.
+    if (nsIPresShell* shell = GetRootDocumentPresShell(content)) {
+        input = input / shell->GetResolution();
+    }
+
+    // Now apply the callback-transform.
     // XXX: technically we need to walk all the way up the layer tree from the layer
     // represented by |aGuid.mScrollId| up to the root of the layer tree and apply
     // the input transforms at each level in turn. However, it is quite difficult
@@ -434,19 +470,9 @@ APZCCallbackHelper::ApplyCallbackTransform(const CSSPoint& aInput,
     // some things transformed improperly. In practice we should rarely hit scenarios
     // where any of this matters, so I'm skipping it for now and just doing the single
     // transform for the layer that the input hit.
-    nsCOMPtr<nsIContent> content = nsLayoutUtils::FindContentFor(aGuid.mScrollId);
-    if (content) {
-        void* property = content->GetProperty(nsGkAtoms::apzCallbackTransform);
-        CSSPoint delta = property ? (*static_cast<CSSPoint*>(property)) : CSSPoint(0.0f, 0.0f);
-        // The delta is in root content document coordinate space while the
-        // aInput point is in root document coordinate space so convert the
-        // delta to root document space before adding it to the aInput point.
-        float resolution = 1.0f;
-        if (nsIPresShell* shell = GetRootContentDocumentPresShellForContent(content)) {
-            resolution = shell->GetResolution();
-        }
-        delta.x = delta.x * resolution;
-        delta.y = delta.y * resolution;
+    void* property = content->GetProperty(nsGkAtoms::apzCallbackTransform);
+    if (property) {
+        CSSPoint delta = (*static_cast<CSSPoint*>(property));
         input += delta;
     }
     return input;

layout/base/nsIPresShell.h

@@ -1416,7 +1416,12 @@ public:
   virtual nsresult SetResolution(float aResolution) = 0;
   float GetResolution() { return mResolution.valueOr(1.0); }
   virtual float GetCumulativeResolution() = 0;
-  virtual float GetCumulativeScaleResolution() = 0;
+
+  /**
+   * Calculate the cumulative scale resolution from this document up to
+   * but not including the root document.
+   */
+  virtual float GetCumulativeNonRootScaleResolution() = 0;
 
   /**
    * Was the current resolution set by the user or just default initialized?

layout/base/nsLayoutUtils.cpp

@@ -2021,7 +2021,7 @@ nsLayoutUtils::GetEventCoordinatesRelativeTo(nsIWidget* aWidget,
       nsPoint pt(presContext->DevPixelsToAppUnits(aPoint.x),
                  presContext->DevPixelsToAppUnits(aPoint.y));
       pt = pt - view->ViewToWidgetOffset();
-      pt = pt.RemoveResolution(presContext->PresShell()->GetCumulativeScaleResolution());
+      pt = pt.RemoveResolution(presContext->PresShell()->GetCumulativeNonRootScaleResolution());
       return pt;
     }
   }
@@ -2060,7 +2060,7 @@ nsLayoutUtils::GetEventCoordinatesRelativeTo(nsIWidget* aWidget,
   nsIPresShell* shell = aFrame->PresContext()->PresShell();
 
   // XXX Bug 1224748 - Update nsLayoutUtils functions to correctly handle nsPresShell resolution
-  widgetToView = widgetToView.RemoveResolution(shell->GetCumulativeScaleResolution());
+  widgetToView = widgetToView.RemoveResolution(shell->GetCumulativeNonRootScaleResolution());
 
   /* If we encountered a transform, we can't do simple arithmetic to figure
    * out how to convert back to aFrame's coordinates and must use the CTM.
@@ -2808,7 +2808,8 @@ nsLayoutUtils::TranslateViewToWidget(nsPresContext* aPresContext,
     return LayoutDeviceIntPoint(NS_UNCONSTRAINEDSIZE, NS_UNCONSTRAINEDSIZE);
   }
 
-  nsPoint pt = (aPt + viewOffset).ApplyResolution(aPresContext->PresShell()->GetCumulativeScaleResolution());
+  nsPoint pt = (aPt +
+  viewOffset).ApplyResolution(aPresContext->PresShell()->GetCumulativeNonRootScaleResolution());
   LayoutDeviceIntPoint relativeToViewWidget(aPresContext->AppUnitsToDevPixels(pt.x),
                                             aPresContext->AppUnitsToDevPixels(pt.y));
   return relativeToViewWidget + WidgetToWidgetOffset(viewWidget, aWidget);

layout/base/nsPresShell.cpp

@@ -5330,13 +5330,16 @@ float PresShell::GetCumulativeResolution()
   return resolution;
 }
 
-float PresShell::GetCumulativeScaleResolution()
+float PresShell::GetCumulativeNonRootScaleResolution()
 {
   float resolution = 1.0;
   nsIPresShell* currentShell = this;
   while (currentShell) {
-    resolution *=  currentShell->ScaleToResolution() ? currentShell->GetResolution() : 1.0f;
-    nsPresContext* parentCtx = currentShell->GetPresContext()->GetParentPresContext();
+    nsPresContext* currentCtx = currentShell->GetPresContext();
+    if (currentCtx != currentCtx->GetRootPresContext()) {
+      resolution *=  currentShell->ScaleToResolution() ? currentShell->GetResolution() : 1.0f;
+    }
+    nsPresContext* parentCtx = currentCtx->GetParentPresContext();
     if (parentCtx) {
       currentShell = parentCtx->PresShell();
     } else {

layout/base/nsPresShell.h

@@ -218,7 +218,7 @@ public:
   }
   virtual bool ScaleToResolution() const override;
   virtual float GetCumulativeResolution() override;
-  virtual float GetCumulativeScaleResolution() override;
+  virtual float GetCumulativeNonRootScaleResolution() override;
 
   //nsIViewObserver interface