Fix bug 'Children of fixed elements do not always remain fixed themselves'

The problem was twofold:
- webkit didn't create composited layers of the children div unless they were
  intersecting with the fixed layer
- the children divs layers are siblings, not children of the fixed layer

The solution is to:
1/ mark layers as needed to be composited if their ancestor is a
   fixed element (in RenderLayerCompositor)
2/ as the GraphicsLayer/LayerAndroid hierarchy is based on the RenderLayer
   hierarchy (z-order..) and not the display hierarchy, we need to a way of
   updating the position of the contained layers when a fixed layer move.

We do that by:
- marking layers contained in a fixed layer as being linked to the
  fixed layer (GraphicsLayerAndroid::syncFixedDescendants), and set the
  offset between the layer and the fixed layer.
- when pushing the layers tree to the UI side, we ensure that such layers
  are linked to their corresponding fixed layer
  (LayerAndroid::ensureFixedLayersForDescendants)
- when we draw, we do a first pass to update the fixed layers position
  (LayerAndroid::updateFixedLayersPositions) then update the rest of the
  layers (LayerAndroid::updatePositions). The layers that are linked
  to the fixed layers will then update their position relative to it,
  using the original offset between the fixed layer and the layer.

Bug:2470701
Change-Id: I512966df94de6a5f84aff335c5d09b3f027bc2c3

1 files changed, 17 insertions, 5 deletions

diff --git a/WebCore/rendering/RenderLayerCompositor.cpp b/WebCore/rendering/RenderLayerCompositor.cpp
index dc9b94d..a831a8b 100644
--- a/WebCore/rendering/RenderLayerCompositor.cpp
+++ b/WebCore/rendering/RenderLayerCompositor.cpp
@@ -702,7 +702,7 @@ void RenderLayerCompositor::rebuildCompositingLayerTree(RenderLayer* layer, cons
             rebuildCompositingLayerTree(curLayer, childState, childList);
         }
     }
-    
+
     if (layer->isStackingContext()) {
         if (Vector<RenderLayer*>* posZOrderList = layer->posZOrderList()) {
             size_t listSize = posZOrderList->size();
@@ -712,7 +712,7 @@ void RenderLayerCompositor::rebuildCompositingLayerTree(RenderLayer* layer, cons
             }
         }
     }
-    
+
     if (layerBacking) {
         layerBacking->parentForSublayers()->setChildren(layerChildren);
         childLayersOfEnclosingLayer.append(layerBacking->childForSuperlayers());
@@ -927,6 +927,17 @@ bool RenderLayerCompositor::needsToBeComposited(const RenderLayer* layer) const
     if (!m_hasAcceleratedCompositing || !layer->isSelfPaintingLayer())
         return false;
 
+#if PLATFORM(ANDROID)
+    // if an ancestor is fixed positionned, we need to be composited...
+    RenderObject* renderer = layer->renderer();
+    RenderObject* parent = renderer->parent();
+    while (parent && (parent = renderer->parent())) {
+      if (parent->isPositioned() && parent->style()->position() == FixedPosition)
+        return true;
+      renderer = parent;
+    }
+#endif
+
     return requiresCompositingLayer(layer) || layer->mustOverlapCompositedLayers();
 }
 
@@ -944,13 +955,14 @@ bool RenderLayerCompositor::requiresCompositingLayer(const RenderLayer* layer) c
     // The root layer always has a compositing layer, but it may not have backing.
     return (inCompositingMode() && layer->isRootLayer()) ||
              requiresCompositingForTransform(renderer) ||
+#if PLATFORM(ANDROID)
+             (renderer->isPositioned() && renderer->style()->position() == FixedPosition) ||
+#else
              requiresCompositingForVideo(renderer) ||
              requiresCompositingForCanvas(renderer) ||
              requiresCompositingForPlugin(renderer) ||
-             renderer->style()->backfaceVisibility() == BackfaceVisibilityHidden ||
-#if PLATFORM(ANDROID)
-             (renderer->isPositioned() && renderer->style()->position() == FixedPosition) ||
 #endif
+             renderer->style()->backfaceVisibility() == BackfaceVisibilityHidden ||
              clipsCompositingDescendants(layer) ||
              requiresCompositingForAnimation(renderer);
 }