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

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

Index: third_party/WebKit/Source/core/paint/PaintLayer.cpp
diff --git a/third_party/WebKit/Source/core/paint/PaintLayer.cpp b/third_party/WebKit/Source/core/paint/PaintLayer.cpp
index e28864f96d0874722932b54f20c660b40fd59c93..2e0fab6cc8e0fab36282462238f5e7788f8a1993 100644
--- a/third_party/WebKit/Source/core/paint/PaintLayer.cpp
+++ b/third_party/WebKit/Source/core/paint/PaintLayer.cpp
@@ -52,6 +52,7 @@
 #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"
@@ -2185,8 +2186,8 @@ LayoutRect PaintLayer::boundingBoxForCompositing(const PaintLayer* ancestorLayer
     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,