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, 6 insertions, 3 deletions

diff --git a/WebKit/android/nav/WebView.cpp b/WebKit/android/nav/WebView.cpp
index 2c6b538..86f8797 100644
--- a/WebKit/android/nav/WebView.cpp
+++ b/WebKit/android/nav/WebView.cpp
@@ -294,7 +294,8 @@ void scrollRectOnScreen(const IntRect& rect)
     calcOurContentVisibleRect(&visible);
 #if USE(ACCELERATED_COMPOSITING)
     if (m_rootLayer) {
-        m_rootLayer->updatePositions(visible);
+        m_rootLayer->updateFixedLayersPositions(visible);
+        m_rootLayer->updatePositions();
         visible = m_rootLayer->subtractLayers(visible);
     }
 #endif
@@ -421,7 +422,8 @@ void drawExtras(SkCanvas* canvas, int extras)
     calcOurContentVisibleRect(&visible);
     // call this to be sure we've adjusted for any scrolling or animations
     // before we actually draw
-    m_rootLayer->updatePositions(visible);
+    m_rootLayer->updateFixedLayersPositions(visible);
+    m_rootLayer->updatePositions();
     m_rootLayer->draw(canvas);
 #endif
 }
@@ -548,7 +550,8 @@ CachedRoot* getFrameCache(FrameCachePermission allowNewer)
         LayerAndroid* layer = const_cast<LayerAndroid*>(
                                                 m_frameCacheUI->rootLayer());
         if (layer) {
-            layer->updatePositions(visible);
+            layer->updateFixedLayersPositions(visible);
+            layer->updatePositions();
         }
     }
 #endif