Fix visual rect for background box painting in composited scrollers.

third_party/WebKit/Source/core/layout/LayoutBoxModelObject.cpp

Index: third_party/WebKit/Source/core/layout/LayoutBoxModelObject.cpp
diff --git a/third_party/WebKit/Source/core/layout/LayoutBoxModelObject.cpp b/third_party/WebKit/Source/core/layout/LayoutBoxModelObject.cpp
index a6f845941eaaee947c68515fe29fc3273ab92a03..d83f1633504c0f04c338ede0370b2395be6c27fd 100644
--- a/third_party/WebKit/Source/core/layout/LayoutBoxModelObject.cpp
+++ b/third_party/WebKit/Source/core/layout/LayoutBoxModelObject.cpp
@@ -439,6 +439,8 @@ void LayoutBoxModelObject::setBackingNeedsPaintInvalidationInRect(const LayoutRe
             // rather than updating the entire rect.
             const LayoutRect& scrollingContentsRect = toLayoutBox(this)->layoutOverflowRect();
             layer()->compositedLayerMapping()->setScrollingContentsNeedDisplayInRect(scrollingContentsRect, invalidationReason, object);
+            layer()->setNeedsRepaint();

[chrishtr, 2016/08/22 17:34:54]

Why is this line needed?

[wkorman, 2016/08/22 17:42:27]

Copying my comment from https://codereview.chromium.org/2254893005/#msg15

We need this due to subsequent invalidateDisplayItemClient() DCHECK:

DCHECK(!paintingLayer() || paintingLayer()->needsRepaint());
https://cs.chromium.org/chromium/src/third_party/WebKit/Source/core/layout/La...

I have concern this change may be introducing over-painting for this particular
scroll/background situation, but note http://crrev.com/2235873002 already
introduced line 440 which invalidates a rect sized to the entire scrolling
contents. But perhaps calling PaintLayer::setNeedsRepaint() ends up acting as an
overly large hammer and I'm unaware? I'll see if this produces diff in repaint
text expectations for other layout tests.

[chrishtr, 2016/08/22 17:46:33]

No it's ok I think. Thanks for the context.

+            invalidateDisplayItemClient(*layer()->compositedLayerMapping()->scrollingContentsLayer(), invalidationReason);
         }
         layer()->compositedLayerMapping()->setNonScrollingContentsNeedDisplayInRect(r, invalidationReason, object);
     } else {