Bug 1061327 - When the scrolled layer is not an ancestor of the scrollbar layer, search the whole layer tree. r=botond

gfx/layers/composite/AsyncCompositionManager.cpp

@@ -744,36 +744,43 @@ ApplyAsyncTransformToScrollbarForContent(Layer* aScrollbar,
 }
 
 static LayerMetricsWrapper
-FindScrolledLayerForScrollbar(Layer* aScrollbar, bool* aOutIsAncestor)
+FindScrolledLayerRecursive(Layer* aScrollbar, const LayerMetricsWrapper& aSubtreeRoot)
 {
-  // XXX: once bug 967844 is implemented there might be multiple scrolled layers
-  // that correspond to the scrollbar's scrollId. Verify that we deal with those
-  // cases correctly.
-
-  // Search all siblings of aScrollbar and of its ancestors.
-  LayerMetricsWrapper scrollbar(aScrollbar, LayerMetricsWrapper::StartAt::BOTTOM);
-  for (LayerMetricsWrapper ancestor = scrollbar; ancestor; ancestor = ancestor.GetParent()) {
-    for (LayerMetricsWrapper scrollTarget = ancestor;
-         scrollTarget;
-         scrollTarget = scrollTarget.GetPrevSibling()) {
-      if (scrollTarget != scrollbar &&
-          LayerIsScrollbarTarget(scrollTarget, aScrollbar)) {
-        *aOutIsAncestor = (scrollTarget == ancestor);
-        return scrollTarget;
-      }
-    }
-    for (LayerMetricsWrapper scrollTarget = ancestor.GetNextSibling();
-         scrollTarget;
-         scrollTarget = scrollTarget.GetNextSibling()) {
-      if (LayerIsScrollbarTarget(scrollTarget, aScrollbar)) {
-        *aOutIsAncestor = false;
-        return scrollTarget;
-      }
+  if (LayerIsScrollbarTarget(aSubtreeRoot, aScrollbar)) {
+    return aSubtreeRoot;
+  }
+  for (LayerMetricsWrapper child = aSubtreeRoot.GetFirstChild(); child;
+         child = child.GetNextSibling()) {
+    LayerMetricsWrapper target = FindScrolledLayerRecursive(aScrollbar, child);
+    if (target) {
+      return target;
     }
   }
   return LayerMetricsWrapper();
 }
 
+static LayerMetricsWrapper
+FindScrolledLayerForScrollbar(Layer* aScrollbar, bool* aOutIsAncestor)
+{
+  // Search ancestors first.
+  LayerMetricsWrapper scrollbar(aScrollbar);
+  for (LayerMetricsWrapper ancestor = scrollbar; ancestor; ancestor = ancestor.GetParent()) {
+    if (LayerIsScrollbarTarget(ancestor, aScrollbar)) {
+      *aOutIsAncestor = true;
+      return ancestor;
+    }
+  }
+
+  // If the scrolled target is not an ancestor, search the whole layer tree.
+  // XXX It would be much better to search the APZC tree instead of the layer
+  // tree. That way we would ignore non-scrollable layers, and we'd only visit
+  // each scroll ID once. In the end we only need the APZC and the FrameMetrics
+  // of the scrolled target.
+  *aOutIsAncestor = false;
+  LayerMetricsWrapper root(aScrollbar->Manager()->GetRoot());
+  return FindScrolledLayerRecursive(aScrollbar, root);
+}
+
 void
 AsyncCompositionManager::ApplyAsyncTransformToScrollbar(Layer* aLayer)
 {