Bug 607417 - Reconcile async scrolling for fixed position layers. r=ajuma

Untranslate fixed position layers when doing async scrolling so that they don't
jump about as content re-renders them in the correct place.

gfx/layers/ipc/CompositorParent.cpp

@@ -166,6 +166,8 @@ void
 CompositorParent::ResumeCompositionAndResize(int width, int height)
 {
   static_cast<LayerManagerOGL*>(mLayerManager.get())->SetSurfaceSize(width, height);
+  mWidgetSize.width = width;
+  mWidgetSize.height = height;
   ResumeComposition();
 }
 
@@ -292,6 +294,33 @@ CompositorParent::GetPrimaryScrollableLayer()
   return root;
 }
 
+static void
+ReverseViewTranslation(gfx3DMatrix& aTransform,
+                       const ViewTransform& aViewTransform)
+{
+  aTransform._41 -= aViewTransform.mTranslation.x / aViewTransform.mXScale;
+  aTransform._42 -= aViewTransform.mTranslation.y / aViewTransform.mYScale;
+}
+
+void
+CompositorParent::UntranslateFixedLayers(Layer* aLayer,
+                                         const ViewTransform& aTransform)
+{
+  if (aLayer->GetIsFixedPosition() &&
+      !aLayer->GetParent()->GetIsFixedPosition()) {
+    gfx3DMatrix layerTransform = aLayer->GetTransform();
+    ReverseViewTranslation(layerTransform, aTransform);
+
+    ShadowLayer* shadow = aLayer->AsShadowLayer();
+    shadow->SetShadowTransform(layerTransform);
+  }
+
+  for (Layer* child = aLayer->GetFirstChild();
+       child; child = child->GetNextSibling()) {
+    UntranslateFixedLayers(child, aTransform);
+  }
+}
+
 // Go down shadow layer tree, setting properties to match their non-shadow
 // counterparts.
 static void
@@ -374,6 +403,17 @@ CompositorParent::TransformShadowTree()
       (mScrollOffset.y / tempScaleDiffY - metricsScrollOffset.y) * mYScale);
     ViewTransform treeTransform(-scrollCompensation, mXScale, mYScale);
     shadow->SetShadowTransform(gfx3DMatrix(treeTransform) * currentTransform);
+
+    // Alter the scroll offset so that fixed position layers remain within
+    // the page area.
+    int offsetX = NS_MAX(0, NS_MIN(mScrollOffset.x, mContentSize.width - mWidgetSize.width));
+    int offsetY = NS_MAX(0, NS_MIN(mScrollOffset.y, mContentSize.height - mWidgetSize.height));
+    treeTransform.mTranslation.x =
+      -(offsetX / tempScaleDiffX - metricsScrollOffset.x) * mXScale;
+    treeTransform.mTranslation.y =
+      -(offsetY / tempScaleDiffY - metricsScrollOffset.y) * mYScale;
+
+    UntranslateFixedLayers(layer, treeTransform);
   } else {
     ViewTransform treeTransform(nsIntPoint(0,0), mXScale, mYScale);
     shadow->SetShadowTransform(gfx3DMatrix(treeTransform) * currentTransform);
@@ -433,14 +473,18 @@ PLayersParent*
 CompositorParent::AllocPLayers(const LayersBackend &backendType)
 {
   if (backendType == LayerManager::LAYERS_OPENGL) {
-#ifdef MOZ_JAVA_COMPOSITOR
     nsIntRect rect;
     mWidget->GetBounds(rect);
+    mWidgetSize.width = rect.width;
+    mWidgetSize.height = rect.height;
+#ifdef MOZ_JAVA_COMPOSITOR
     nsRefPtr<LayerManagerOGL> layerManager =
       new LayerManagerOGL(mWidget, rect.width, rect.height, true);
 #else
     nsRefPtr<LayerManagerOGL> layerManager = new LayerManagerOGL(mWidget);
 #endif
+    // mWidget doesn't belong to the compositor thread, so set it to NULL here
+    // to avoid accessing it.
     mWidget = NULL;
     mLayerManager = layerManager;
 

gfx/layers/ipc/CompositorParent.h

@@ -137,6 +137,13 @@ private:
    */
   Layer* GetPrimaryScrollableLayer();
 
+  /**
+   * Recursively reverses the translation portion of the given ViewTransform on
+   * all fixed position layers that aren't children of other fixed position
+   * layers.
+   */
+  void UntranslateFixedLayers(Layer* aLayer, const ViewTransform& aTransform);
+
   nsRefPtr<LayerManager> mLayerManager;
   nsIWidget* mWidget;
   CancelableTask *mCurrentCompositeTask;
@@ -150,6 +157,7 @@ private:
   float mYScale;
   nsIntPoint mScrollOffset;
   nsIntSize mContentSize;
+  nsIntSize mWidgetSize;
 
   // When this flag is set, the next composition will be the first for a
   // particular document (i.e. the document displayed on the screen will change).

mobile/android/base/GeckoAppShell.java

@@ -411,6 +411,9 @@ public class GeckoAppShell
         f = Environment.getDownloadCacheDirectory();
         GeckoAppShell.putenv("EXTERNAL_STORAGE=" + f.getPath());
 
+        // Enable fixed position layers
+        GeckoAppShell.putenv("MOZ_ENABLE_FIXED_POSITION_LAYERS=1");
+
         putLocaleEnv();
     }