Root layer scrolling: set correct size for document's main GraphicsLayer.

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 34 matching lines @@
 #include "core/paint/PaintLayer.h"
 
 #include "core/CSSPropertyNames.h"
 #include "core/HTMLNames.h"
 #include "core/css/PseudoStyleRequest.h"
 #include "core/dom/Document.h"
 #include "core/dom/shadow/ShadowRoot.h"
 #include "core/frame/DeprecatedScheduleStyleRecalcDuringLayout.h"
 #include "core/frame/FrameView.h"
 #include "core/frame/LocalFrame.h"
+#include "core/frame/Settings.h"
 #include "core/html/HTMLFrameElement.h"
 #include "core/layout/HitTestRequest.h"
 #include "core/layout/HitTestResult.h"
 #include "core/layout/HitTestingTransformState.h"
 #include "core/layout/LayoutFlowThread.h"
 #include "core/layout/LayoutGeometryMap.h"
 #include "core/layout/LayoutInline.h"
 #include "core/layout/LayoutPart.h"
 #include "core/layout/LayoutReplica.h"
 #include "core/layout/LayoutScrollbar.h"
@@ skipping 2113 matching lines @@
     if (!isSelfPaintingLayer())
         return LayoutRect();
 
     if (!ancestorLayer)
         ancestorLayer = this;
 
     // FIXME: This could be improved to do a check like hasVisibleNonCompositingDescendantLayers() (bug 92580).
     if (this != ancestorLayer && !hasVisibleContent() && !hasVisibleDescendant())
         return LayoutRect();
 
-    // The root layer is always just the size of the document.
-    if (isRootLayer())
+    // Without composited scrolling, the root layer is the size of the document.
+    if (isRootLayer() && !needsCompositedScrolling())

[Ian Vollick, 2015/12/22 03:21:50]

If the root layer does need composited scrolling, would it be correct to return
the viewRect here? I.e., can the code here match that in ViewPainter (and can it
be shared)?

[skobes, 2015/12/28 23:33:17]

It does return the viewRect, indirectly, since the result of localClipRect() is
computed by PaintLayerClipper::calculateRects, which was updated to use
viewRect() in http://crrev.com/1262063002.

I've updated ViewPainter to call PaintLayer::boundingBoxForCompositing so that
this logic is reused.

[Ian Vollick, 2015/12/29 00:54:29]

Thanks, lgtm.

         return LayoutRect(m_layoutObject->view()->documentRect());
 
     // The layer created for the LayoutFlowThread is just a helper for painting and hit-testing,
     // and should not contribute to the bounding box. The LayoutMultiColumnSets will contribute
     // the correct size for the layout content of the multicol container.
     if (layoutObject()->isLayoutFlowThread())
         return LayoutRect();
 
     // If there is a clip applied by an ancestor to this PaintLayer but below or equal to |ancestorLayer|,
     // use that clip as the bounds rather than the recursive bounding boxes, since the latter may be larger than the
@@ skipping 602 matching lines @@
 
 void showLayerTree(const blink::LayoutObject* layoutObject)
 {
     if (!layoutObject) {
         fprintf(stderr, "Cannot showLayerTree. Root is (nil)\n");
         return;
     }
     showLayerTree(layoutObject->enclosingLayer());
 }
 #endif