Fix pixel snapping issues when layers become composited

Source/core/rendering/RenderLayer.cpp

Index: Source/core/rendering/RenderLayer.cpp
diff --git a/Source/core/rendering/RenderLayer.cpp b/Source/core/rendering/RenderLayer.cpp
index 0eef2763bebe2734f6a1a5304ccb42bf5c01374b..8edbc8630eeedec3d09686cf47ae61af9c468d31 100644
--- a/Source/core/rendering/RenderLayer.cpp
+++ b/Source/core/rendering/RenderLayer.cpp
@@ -2016,7 +2016,10 @@ void RenderLayer::paintLayerContents(GraphicsContext* context, const LayerPainti
     LayoutPoint offsetFromRoot;
     convertToLayerCoords(paintingInfo.rootLayer, offsetFromRoot);
 
-    IntRect rootRelativeBounds;
+    if (compositingState() == PaintsIntoOwnBacking)
+        offsetFromRoot.move(m_compositedLayerMapping->subpixelAccumulation());
+
+    LayoutRect rootRelativeBounds;
     bool rootRelativeBoundsComputed = false;
 
     // Apply clip-path to context.
@@ -2138,7 +2141,7 @@ void RenderLayer::paintLayerContents(GraphicsContext* context, const LayerPainti
         // fragment should paint.
         collectFragments(layerFragments, localPaintingInfo.rootLayer, localPaintingInfo.region, localPaintingInfo.paintDirtyRect,
             (paintFlags & PaintLayerTemporaryClipRects) ? TemporaryClipRects : PaintingClipRects, IgnoreOverlayScrollbarSize,
-            (isPaintingOverflowContents) ? IgnoreOverflowClip : RespectOverflowClip, &offsetFromRoot);
+            (isPaintingOverflowContents) ? IgnoreOverflowClip : RespectOverflowClip, &offsetFromRoot, localPaintingInfo.subPixelAccumulation);
         updatePaintingInfoForFragments(layerFragments, localPaintingInfo, paintFlags, shouldPaintContent, &offsetFromRoot);
     }
 
@@ -2244,12 +2247,12 @@ void RenderLayer::paintChildren(unsigned childrenToVisit, GraphicsContext* conte
 
 void RenderLayer::collectFragments(LayerFragments& fragments, const RenderLayer* rootLayer, RenderRegion* region, const LayoutRect& dirtyRect,
     ClipRectsType clipRectsType, OverlayScrollbarSizeRelevancy inOverlayScrollbarSizeRelevancy, ShouldRespectOverflowClip respectOverflowClip, const LayoutPoint* offsetFromRoot,
-    const LayoutRect* layerBoundingBox)
+    const LayoutSize& subPixelAccumulation, const LayoutRect* layerBoundingBox)
 {
     if (!enclosingPaginationLayer() || hasTransform()) {
         // For unpaginated layers, there is only one fragment.
         LayerFragment fragment;
-        ClipRectsContext clipRectsContext(rootLayer, region, clipRectsType, inOverlayScrollbarSizeRelevancy, respectOverflowClip);
+        ClipRectsContext clipRectsContext(rootLayer, region, clipRectsType, inOverlayScrollbarSizeRelevancy, respectOverflowClip, subPixelAccumulation);
         clipper().calculateRects(clipRectsContext, dirtyRect, fragment.layerBounds, fragment.backgroundRect, fragment.foregroundRect, fragment.outlineRect, offsetFromRoot);
         fragments.append(fragment);
         return;
@@ -2335,7 +2338,7 @@ void RenderLayer::paintTransformedLayerIntoFragments(GraphicsContext* context, c
     LayoutRect transformedExtent = transparencyClipBox(this, enclosingPaginationLayer(), PaintingTransparencyClipBox, RootOfTransparencyClipBox, paintingInfo.paintBehavior);
     enclosingPaginationLayer()->collectFragments(enclosingPaginationFragments, paintingInfo.rootLayer, paintingInfo.region, paintingInfo.paintDirtyRect,
         (paintFlags & PaintLayerTemporaryClipRects) ? TemporaryClipRects : PaintingClipRects, IgnoreOverlayScrollbarSize,
-        (paintFlags & PaintLayerPaintingOverflowContents) ? IgnoreOverflowClip : RespectOverflowClip, &offsetOfPaginationLayerFromRoot, &transformedExtent);
+        (paintFlags & PaintLayerPaintingOverflowContents) ? IgnoreOverflowClip : RespectOverflowClip, &offsetOfPaginationLayerFromRoot, paintingInfo.subPixelAccumulation, &transformedExtent);
 
     for (size_t i = 0; i < enclosingPaginationFragments.size(); ++i) {
         const LayerFragment& fragment = enclosingPaginationFragments.at(i);
@@ -2361,6 +2364,15 @@ void RenderLayer::paintTransformedLayerIntoFragments(GraphicsContext* context, c
     }
 }
 
+static inline LayoutSize subPixelAccumulationIfNeeded(const LayoutSize& subPixelAccumulation, CompositingState compositingState)
+{
+    // Only apply the sub-pixel accumulation if we don't paint into our own backing layer, otherwise the position
+    // of the renderer already includes any sub-pixel offset.
+    if (compositingState == PaintsIntoOwnBacking)
+        return LayoutSize();
+    return subPixelAccumulation;
+}
+
 void RenderLayer::paintBackgroundForFragments(const LayerFragments& layerFragments, GraphicsContext* context, GraphicsContext* transparencyLayerContext,
     const LayoutRect& transparencyPaintDirtyRect, bool haveTransparency, const LayerPaintingInfo& localPaintingInfo, PaintBehavior paintBehavior,
     RenderObject* paintingRootForRenderer)
@@ -2383,7 +2395,7 @@ void RenderLayer::paintBackgroundForFragments(const LayerFragments& layerFragmen
         // Paint the background.
         // FIXME: Eventually we will collect the region from the fragment itself instead of just from the paint info.
         PaintInfo paintInfo(context, pixelSnappedIntRect(fragment.backgroundRect.rect()), PaintPhaseBlockBackground, paintBehavior, paintingRootForRenderer, localPaintingInfo.region, 0, 0, localPaintingInfo.rootLayer->renderer());
-        renderer()->paint(paintInfo, toPoint(fragment.layerBounds.location() - renderBoxLocation() + localPaintingInfo.subPixelAccumulation));
+        renderer()->paint(paintInfo, toPoint(fragment.layerBounds.location() - renderBoxLocation() + subPixelAccumulationIfNeeded(localPaintingInfo.subPixelAccumulation, compositingState())));
 
         if (localPaintingInfo.clipToDirtyRect)
             restoreClip(context, localPaintingInfo.paintDirtyRect, fragment.backgroundRect);
@@ -2443,7 +2455,7 @@ void RenderLayer::paintForegroundForFragmentsWithPhase(PaintPhase phase, const L
         PaintInfo paintInfo(context, pixelSnappedIntRect(fragment.foregroundRect.rect()), phase, paintBehavior, paintingRootForRenderer, localPaintingInfo.region, 0, 0, localPaintingInfo.rootLayer->renderer());
         if (phase == PaintPhaseForeground)
             paintInfo.overlapTestRequests = localPaintingInfo.overlapTestRequests;
-        renderer()->paint(paintInfo, toPoint(fragment.layerBounds.location() - renderBoxLocation() + localPaintingInfo.subPixelAccumulation));
+        renderer()->paint(paintInfo, toPoint(fragment.layerBounds.location() - renderBoxLocation() + subPixelAccumulationIfNeeded(localPaintingInfo.subPixelAccumulation, compositingState())));
 
         if (shouldClip)
             restoreClip(context, localPaintingInfo.paintDirtyRect, fragment.foregroundRect);
@@ -2461,7 +2473,7 @@ void RenderLayer::paintOutlineForFragments(const LayerFragments& layerFragments,
         // Paint our own outline
         PaintInfo paintInfo(context, pixelSnappedIntRect(fragment.outlineRect.rect()), PaintPhaseSelfOutline, paintBehavior, paintingRootForRenderer, localPaintingInfo.region, 0, 0, localPaintingInfo.rootLayer->renderer());
         clipToRect(localPaintingInfo.rootLayer, context, localPaintingInfo.paintDirtyRect, fragment.outlineRect, DoNotIncludeSelfForBorderRadius);
-        renderer()->paint(paintInfo, toPoint(fragment.layerBounds.location() - renderBoxLocation() + localPaintingInfo.subPixelAccumulation));
+        renderer()->paint(paintInfo, toPoint(fragment.layerBounds.location() - renderBoxLocation() + subPixelAccumulationIfNeeded(localPaintingInfo.subPixelAccumulation, compositingState())));
         restoreClip(context, localPaintingInfo.paintDirtyRect, fragment.outlineRect);
     }
 }
@@ -2480,7 +2492,7 @@ void RenderLayer::paintMaskForFragments(const LayerFragments& layerFragments, Gr
         // Paint the mask.
         // FIXME: Eventually we will collect the region from the fragment itself instead of just from the paint info.
         PaintInfo paintInfo(context, pixelSnappedIntRect(fragment.backgroundRect.rect()), PaintPhaseMask, PaintBehaviorNormal, paintingRootForRenderer, localPaintingInfo.region, 0, 0, localPaintingInfo.rootLayer->renderer());
-        renderer()->paint(paintInfo, toPoint(fragment.layerBounds.location() - renderBoxLocation() + localPaintingInfo.subPixelAccumulation));
+        renderer()->paint(paintInfo, toPoint(fragment.layerBounds.location() - renderBoxLocation() + subPixelAccumulationIfNeeded(localPaintingInfo.subPixelAccumulation, compositingState())));
 
         if (localPaintingInfo.clipToDirtyRect)
             restoreClip(context, localPaintingInfo.paintDirtyRect, fragment.backgroundRect);
@@ -2500,7 +2512,7 @@ void RenderLayer::paintChildClippingMaskForFragments(const LayerFragments& layer
 
         // Paint the the clipped mask.
         PaintInfo paintInfo(context, pixelSnappedIntRect(fragment.backgroundRect.rect()), PaintPhaseClippingMask, PaintBehaviorNormal, paintingRootForRenderer, localPaintingInfo.region, 0, 0, localPaintingInfo.rootLayer->renderer());
-        renderer()->paint(paintInfo, toPoint(fragment.layerBounds.location() - renderBoxLocation() + localPaintingInfo.subPixelAccumulation));
+        renderer()->paint(paintInfo, toPoint(fragment.layerBounds.location() - renderBoxLocation() + subPixelAccumulationIfNeeded(localPaintingInfo.subPixelAccumulation, compositingState())));
 
         if (localPaintingInfo.clipToDirtyRect)
             restoreClip(context, localPaintingInfo.paintDirtyRect, fragment.foregroundRect);
@@ -2513,7 +2525,7 @@ void RenderLayer::paintOverflowControlsForFragments(const LayerFragments& layerF
         const LayerFragment& fragment = layerFragments.at(i);
         clipToRect(localPaintingInfo.rootLayer, context, localPaintingInfo.paintDirtyRect, fragment.backgroundRect);
         if (RenderLayerScrollableArea* scrollableArea = this->scrollableArea())
-            scrollableArea->paintOverflowControls(context, roundedIntPoint(toPoint(fragment.layerBounds.location() - renderBoxLocation() + localPaintingInfo.subPixelAccumulation)), pixelSnappedIntRect(fragment.backgroundRect.rect()), true);
+            scrollableArea->paintOverflowControls(context, roundedIntPoint(toPoint(fragment.layerBounds.location() - renderBoxLocation() + subPixelAccumulationIfNeeded(localPaintingInfo.subPixelAccumulation, compositingState()))), pixelSnappedIntRect(fragment.backgroundRect.rect()), true);
         restoreClip(context, localPaintingInfo.paintDirtyRect, fragment.backgroundRect);
     }
 }
@@ -2974,7 +2986,7 @@ RenderLayer* RenderLayer::hitTestTransformedLayerInFragments(RenderLayer* rootLa
     LayoutPoint offsetOfPaginationLayerFromRoot;
     LayoutRect transformedExtent = transparencyClipBox(this, enclosingPaginationLayer(), HitTestingTransparencyClipBox, RootOfTransparencyClipBox);
     enclosingPaginationLayer()->collectFragments(enclosingPaginationFragments, rootLayer, hitTestLocation.region(), hitTestRect,
-        RootRelativeClipRects, IncludeOverlayScrollbarSize, RespectOverflowClip, &offsetOfPaginationLayerFromRoot, &transformedExtent);
+        RootRelativeClipRects, IncludeOverlayScrollbarSize, RespectOverflowClip, &offsetOfPaginationLayerFromRoot, LayoutSize(), &transformedExtent);
 
     for (int i = enclosingPaginationFragments.size() - 1; i >= 0; --i) {
         const LayerFragment& fragment = enclosingPaginationFragments.at(i);
@@ -3421,14 +3433,14 @@ IntRect RenderLayer::absoluteBoundingBox() const
     return pixelSnappedIntRect(boundingBox(root()));
 }
 
-IntRect RenderLayer::calculateLayerBounds(const RenderLayer* ancestorLayer, const LayoutPoint* offsetFromRoot, CalculateLayerBoundsFlags flags) const
+LayoutRect RenderLayer::calculateLayerBounds(const RenderLayer* ancestorLayer, const LayoutPoint* offsetFromRoot, CalculateLayerBoundsFlags flags) const
 {
     if (!isSelfPaintingLayer())
-        return IntRect();
+        return LayoutRect();
 
     // FIXME: This could be improved to do a check like hasVisibleNonCompositingDescendantLayers() (bug 92580).
     if ((flags & ExcludeHiddenDescendants) && this != ancestorLayer && !hasVisibleContent() && !hasVisibleDescendant())
-        return IntRect();
+        return LayoutRect();
 
     RenderLayerModelObject* renderer = this->renderer();
 
@@ -3469,7 +3481,7 @@ IntRect RenderLayer::calculateLayerBounds(const RenderLayer* ancestorLayer, cons
             LayoutPoint ancestorRelOffset;
             convertToLayerCoords(ancestorLayer, ancestorRelOffset);
             localClipRect.moveBy(ancestorRelOffset);
-            return pixelSnappedIntRect(localClipRect);
+            return localClipRect;
         }
     }
 
@@ -3481,7 +3493,7 @@ IntRect RenderLayer::calculateLayerBounds(const RenderLayer* ancestorLayer, cons
     if (m_reflectionInfo) {
         RenderLayer* reflectionLayer = m_reflectionInfo->reflectionLayer();
         if (!reflectionLayer->hasCompositedLayerMapping()) {
-            IntRect childUnionBounds = reflectionLayer->calculateLayerBounds(this, 0, descendantFlags);
+            LayoutRect childUnionBounds = reflectionLayer->calculateLayerBounds(this, 0, descendantFlags);
             unionBounds.unite(childUnionBounds);
         }
     }
@@ -3506,7 +3518,7 @@ IntRect RenderLayer::calculateLayerBounds(const RenderLayer* ancestorLayer, cons
         }
 
         if (flags & IncludeCompositedDescendants || !node->layer()->hasCompositedLayerMapping()) {
-            IntRect childUnionBounds = node->layer()->calculateLayerBounds(this, 0, descendantFlags);
+            LayoutRect childUnionBounds = node->layer()->calculateLayerBounds(this, 0, descendantFlags);
             unionBounds.unite(childUnionBounds);
         }
     }
@@ -3530,7 +3542,7 @@ IntRect RenderLayer::calculateLayerBounds(const RenderLayer* ancestorLayer, cons
         convertToLayerCoords(ancestorLayer, ancestorRelOffset);
     unionBounds.moveBy(ancestorRelOffset);
 
-    return pixelSnappedIntRect(unionBounds);
+    return unionBounds;
 }
 
 CompositingState RenderLayer::compositingState() const