Promote -ve z-index fixed/scrollable elements to layers

We normally do not necessarily promote fixed or scrollable
elements to compositing layers to try and reduce the number
of layers that get created. However in the case that such
an element appears in the negative z-order, we must do
the promotion to ensure that the layer is painted at the
correct z-index in the hierarchy (i.e not too high - sites
like to use a fixed image with z-index:-1 to implement
a fixed background -- without this patch we end up painting
the background over the main page content).

Bug: 5152567

Change-Id: I36bfe14660a33d45ad54552ae41b9d7157f8dabf

1 files changed, 11 insertions, 0 deletions

diff --git a/Source/WebCore/rendering/RenderLayerCompositor.cpp b/Source/WebCore/rendering/RenderLayerCompositor.cpp
index 4f4f553..598926d 100644
--- a/Source/WebCore/rendering/RenderLayerCompositor.cpp
+++ b/Source/WebCore/rendering/RenderLayerCompositor.cpp
@@ -764,7 +764,18 @@ void RenderLayerCompositor::computeCompositingRequirements(RenderLayer* layer, O
 
                 // If we have to make a layer for this child, make one now so we can have a contents layer
                 // (since we need to ensure that the -ve z-order child renders underneath our contents).
+#ifdef ANDROID
+                // Normally we try to reduce the number of layers by not promoting all fixed
+                // or scrollable elements to their own compositing layer. But in the case that
+                // we have such an element in the negative z-order, we must make it a layer
+                // otherwise the content will be painted at a higher z-index. This breaks pages
+                // that set a large image with a z-index of -1 to implement a background image,
+                // for example.
+                bool childRequiresCompositing = childState.m_hasFixedElement || childState.m_hasScrollableElement;
+                if (!willBeComposited && (childState.m_subtreeIsCompositing || childRequiresCompositing)) {
+#else
                 if (!willBeComposited && childState.m_subtreeIsCompositing) {
+#endif
                     // make layer compositing
                     layer->setMustOverlapCompositedLayers(true);
                     childState.m_compositingAncestor = layer;