Flash of garbage pixels when playing a show on Hulu

<https://webkit.org/b/119871>

Source/WebCore:

Reviewed by Tim Horton.

When starting a show on Hulu, there's a layer that gets marked as
opaque because it has a child RenderLayer whose background fills the
compositing layer. That child RenderLayer was recently position:fixed
but outside the viewport, so its viewportConstrainedNotCompositedReason
was set to a non-zero value.

However, we failed to clear the viewportConstrainedNotCompositedReason
when the layer became non-fixed. This caused painting the opaque
layer to bail in RenderLayer::paintLayer(), leaving garbage.

Test: compositing/contents-opaque/fixed-to-nonfixed.html

* rendering/RenderLayer.cpp:
(WebCore::RenderLayer::paintLayer): Add an assertion to catch this error in future.
* rendering/RenderLayerCompositor.cpp:
(WebCore::RenderLayerCompositor::updateBacking): Make sure we clear the
ViewportConstrainedNotCompositedReason if the layer is no longer fixed.

Change-Id: Ie1aa935b8271cd8cc3159f5337d483a77d74b8a0
git-svn-id: http://svn.webkit.org/repository/webkit/trunk@154147 268f45cc-cd09-0410-ab3c-d52691b4dbfc
Reviewed-by: Michael Bruning <michael.bruning@digia.com>

2 files changed, 3 insertions, 1 deletions

diff --git a/Source/WebCore/rendering/RenderLayer.cpp b/Source/WebCore/rendering/RenderLayer.cpp
index c378f18e6..2d0b56e79 100644
--- a/Source/WebCore/rendering/RenderLayer.cpp
+++ b/Source/WebCore/rendering/RenderLayer.cpp
@@ -3619,6 +3619,7 @@ void RenderLayer::paintLayer(GraphicsContext* context, const LayerPaintingInfo&
     } else if (viewportConstrainedNotCompositedReason() == NotCompositedForBoundsOutOfView) {
         // Don't paint out-of-view viewport constrained layers (when doing prepainting) because they will never be visible
         // unless their position or viewport size is changed.
+        ASSERT(renderer()->style()->position() == FixedPosition);
         return;
     }
 #endif
diff --git a/Source/WebCore/rendering/RenderLayerCompositor.cpp b/Source/WebCore/rendering/RenderLayerCompositor.cpp
index 6f1fd3e0b..784ebdc80 100644
--- a/Source/WebCore/rendering/RenderLayerCompositor.cpp
+++ b/Source/WebCore/rendering/RenderLayerCompositor.cpp
@@ -767,7 +767,8 @@ bool RenderLayerCompositor::updateBacking(RenderLayer* layer, CompositingChangeR
             if (ScrollingCoordinator* scrollingCoordinator = this->scrollingCoordinator())
                 scrollingCoordinator->frameViewFixedObjectsDidChange(m_renderView->frameView());
         }
-    }
+    } else
+        layer->setViewportConstrainedNotCompositedReason(RenderLayer::NoNotCompositedReason);
     
     if (layer->backing())
         layer->backing()->updateDebugIndicators(m_showDebugBorders, m_showRepaintCounter);