Disallow fragmentation of composited content.

third_party/WebKit/Source/core/paint/PaintLayer.cpp

 /*
  * Copyright (C) 2006, 2007, 2008, 2009, 2010, 2011, 2012 Apple Inc. All rights reserved.
  *
  * Portions are Copyright (C) 1998 Netscape Communications Corporation.
  *
  * Other contributors:
  *   Robert O'Callahan <roc+@cs.cmu.edu>
  *   David Baron <dbaron@fas.harvard.edu>
  *   Christian Biesinger <cbiesinger@web.de>
  *   Randall Jesup <rjesup@wgate.com>
@@ skipping 1472 matching lines @@
 void PaintLayer::appendSingleFragmentIgnoringPagination(PaintLayerFragments& fragments, const PaintLayer* rootLayer, const LayoutRect& dirtyRect, ClipRectsCacheSlot clipRectsCacheSlot, OverlayScrollbarClipBehavior overlayScrollbarClipBehavior, ShouldRespectOverflowClipType respectOverflowClip, const LayoutPoint* offsetFromRoot, const LayoutSize& subPixelAccumulation)
 {
     PaintLayerFragment fragment;
     ClipRectsContext clipRectsContext(rootLayer, clipRectsCacheSlot, overlayScrollbarClipBehavior, subPixelAccumulation);
     if (respectOverflowClip == IgnoreOverflowClip)
         clipRectsContext.setIgnoreOverflowClip();
     clipper().calculateRects(clipRectsContext, dirtyRect, fragment.layerBounds, fragment.backgroundRect, fragment.foregroundRect, offsetFromRoot);
     fragments.append(fragment);
 }
 
+bool PaintLayer::shouldFragmentCompositedBounds(const PaintLayer* compositingLayer) const
+{
+    return enclosingPaginationLayer() && !compositingLayer->enclosingPaginationLayer();

[wkorman, 2016/06/02 18:04:15]

Considering whether a comment to explain this further is warranted here or
elsewhere, re:

https://groups.google.com/a/chromium.org/d/msg/paint-dev/uQF-DlRlf1k/EDXipWUO...

[chrishtr, 2016/06/02 18:22:43]

Done.

+}
+
 void PaintLayer::collectFragments(PaintLayerFragments& fragments, const PaintLayer* rootLayer, const LayoutRect& dirtyRect,
     ClipRectsCacheSlot clipRectsCacheSlot, OverlayScrollbarClipBehavior overlayScrollbarClipBehavior, ShouldRespectOverflowClipType respectOverflowClip, const LayoutPoint* offsetFromRoot,
     const LayoutSize& subPixelAccumulation, const LayoutRect* layerBoundingBox)
 {
     if (!enclosingPaginationLayer()) {
         // For unpaginated layers, there is only one fragment.
         appendSingleFragmentIgnoringPagination(fragments, rootLayer, dirtyRect, clipRectsCacheSlot, overlayScrollbarClipBehavior, respectOverflowClip, offsetFromRoot, subPixelAccumulation);
         return;
     }
 
+    if (!shouldFragmentCompositedBounds(rootLayer)) {
+        appendSingleFragmentIgnoringPagination(fragments, rootLayer, dirtyRect, clipRectsCacheSlot, overlayScrollbarClipBehavior, respectOverflowClip, offsetFromRoot, subPixelAccumulation);
+        return;
+    }
+
     // Compute our offset within the enclosing pagination layer.
     LayoutPoint offsetWithinPaginatedLayer;
     convertToLayerCoords(enclosingPaginationLayer(), offsetWithinPaginatedLayer);
 
     // Calculate clip rects relative to the enclosingPaginationLayer. The purpose of this call is to determine our bounds clipped to intermediate
     // layers between us and the pagination context. It's important to minimize the number of fragments we need to create and this helps with that.
     ClipRectsContext paginationClipRectsContext(enclosingPaginationLayer(), clipRectsCacheSlot, overlayScrollbarClipBehavior);
     if (respectOverflowClip == IgnoreOverflowClip)
         paginationClipRectsContext.setIgnoreOverflowClip();
     LayoutRect layerBoundsInFlowThread;
@@ skipping 733 matching lines @@
 
         // Only enlarge by the filter outsets if we know the filter is going to be rendered in software.
         // Accelerated filters will handle their own outsets.
         if (paintsWithFilters())
             result = mapLayoutRectForFilter(result);
     }
 
     if (transform() && paintsWithTransform(GlobalPaintNormalPhase) && (this != ancestorLayer || options == MaybeIncludeTransformForAncestorLayer))
         result = transform()->mapRect(result);
 
-    if (enclosingPaginationLayer()) {
+    if (shouldFragmentCompositedBounds(ancestorLayer)) {
         convertFromFlowThreadToVisualBoundingBoxInAncestor(ancestorLayer, result);
         return result;
     }
     LayoutPoint delta;
     convertToLayerCoords(ancestorLayer, delta);
     result.moveBy(delta);
     return result;
 }
 
 CompositingState PaintLayer::compositingState() const
@@ skipping 649 matching lines @@
 
 void showLayerTree(const blink::LayoutObject* layoutObject)
 {
     if (!layoutObject) {
         fprintf(stderr, "Cannot showLayerTree. Root is (nil)\n");
         return;
     }
     showLayerTree(layoutObject->enclosingLayer());
 }
 #endif