Bug 1467873 - Fix FrameMetrics::CalculateCompositedRectInCssPixels(). r=kats

- Fix the implementation to return a numerically correct result
  - Change call sites that should have instead been using
    CalculateCompositedSizeInCssPixels(), to do so
  - Rename and document to emphasize that the returned rect is in the
    CSS pixels of the content surrounding the scroll frame

MozReview-Commit-ID: GCPbA1k88rz

--HG--
extra : rebase_source : f5b937936fb84b3ae414d6cf0250add4078a9d53

gfx/layers/FrameMetrics.h

@@ -197,12 +197,25 @@ public:
     return mCompositionBounds.Size() / GetZoom();
   }
 
-  CSSRect CalculateCompositedRectInCssPixels() const
+  /*
+   * Calculate the composition bounds of this frame in the CSS pixels of
+   * the content surrounding the scroll frame. (This can be thought of as
+   * "parent CSS" pixels).
+   * Note that it does not make to ask for the composition bounds in the
+   * CSS pixels of the scrolled content (that is, regular CSS pixels),
+   * because the origin of the composition bounds is not meaningful in that
+   * coordinate space. (The size is, use CalculateCompositedSizeInCssPixels()
+   * for that.)
+   */
+  CSSRect CalculateCompositionBoundsInCssPixelsOfSurroundingContent() const
   {
     if (GetZoom() == CSSToParentLayerScale2D(0, 0)) {
       return CSSRect();  // avoid division by zero
     }
-    return mCompositionBounds / GetZoom();
+    // The CSS pixels of the scrolled content and the CSS pixels of the
+    // surrounding content only differ if the scrolled content is rendered
+    // at a higher resolution, and the difference is the resolution.
+    return mCompositionBounds / GetZoom() * CSSToCSSScale{mPresShellResolution};
   }
 
   CSSSize CalculateBoundedCompositedSizeInCssPixels() const

gfx/layers/apz/src/AsyncPanZoomController.cpp

@@ -1056,7 +1056,7 @@ nsEventStatus AsyncPanZoomController::HandleDragEvent(const MouseInput& aEvent,
     GetAxisStart(direction, mFrameMetrics.GetScrollableRect().TopLeft());
   CSSCoord maxScrollPosition =
     GetAxisStart(direction, mFrameMetrics.GetScrollableRect().BottomRight()) -
-    GetAxisLength(direction, mFrameMetrics.CalculateCompositedRectInCssPixels());
+    GetAxisLength(direction, mFrameMetrics.CalculateCompositionBoundsInCssPixelsOfSurroundingContent());
   CSSCoord scrollPosition = minScrollPosition + (scrollPercent * (maxScrollPosition - minScrollPosition));
 
   scrollPosition = std::max(scrollPosition, minScrollPosition);
@@ -1750,7 +1750,7 @@ AsyncPanZoomController::ConvertScrollbarPoint(const ParentLayerPoint& aScrollbar
   scrollbarPoint = scrollbarPoint * mFrameMetrics.GetPresShellResolution();
 
   // Now, get it to be relative to the beginning of the scroll track.
-  CSSRect cssCompositionBound = mFrameMetrics.CalculateCompositedRectInCssPixels();
+  CSSRect cssCompositionBound = mFrameMetrics.CalculateCompositionBoundsInCssPixelsOfSurroundingContent();
   return GetAxisStart(*aThumbData.mDirection, scrollbarPoint)
       - GetAxisStart(*aThumbData.mDirection, cssCompositionBound)
       - aThumbData.mScrollTrackStart;

gfx/layers/apz/util/APZCCallbackHelper.cpp

@@ -210,10 +210,10 @@ SetDisplayPortMargins(nsIPresShell* aPresShell,
         aContent->GetPrimaryFrame(), nsLayoutUtils::RepaintMode::Repaint);
   }
 
-  CSSRect baseCSS = aMetrics.CalculateCompositedRectInCssPixels();
+  CSSSize baseSize = aMetrics.CalculateCompositedSizeInCssPixels();
   nsRect base(0, 0,
-              baseCSS.Width() * nsPresContext::AppUnitsPerCSSPixel(),
-              baseCSS.Height() * nsPresContext::AppUnitsPerCSSPixel());
+              baseSize.width * nsPresContext::AppUnitsPerCSSPixel(),
+              baseSize.height * nsPresContext::AppUnitsPerCSSPixel());
   nsLayoutUtils::SetDisplayPortBaseIfNotSet(aContent, base);
 }
 

layout/base/Units.h

@@ -139,6 +139,7 @@ typedef gfx::IntSizeTyped<DesktopPixel> DesktopIntSize;
 typedef gfx::RectTyped<DesktopPixel> DesktopRect;
 typedef gfx::IntRectTyped<DesktopPixel> DesktopIntRect;
 
+typedef gfx::ScaleFactor<CSSPixel, CSSPixel> CSSToCSSScale;
 typedef gfx::ScaleFactor<CSSPixel, LayoutDevicePixel> CSSToLayoutDeviceScale;
 typedef gfx::ScaleFactor<CSSPixel, LayerPixel> CSSToLayerScale;
 typedef gfx::ScaleFactor<CSSPixel, ScreenPixel> CSSToScreenScale;