From 709cb5dac481176a484423daa6e07d28370691a7 Mon Sep 17 00:00:00 2001
From: Markus Stange <mstange@themasta.com>
Date: Wed, 28 May 2014 16:14:43 +0200
Subject: [PATCH] Bug 1013392 - Adjust the search algorithm for async scrollbar
 thumb shifting. r=kats

---
 .../composite/AsyncCompositionManager.cpp     | 56 ++++++++++++-------
 1 file changed, 37 insertions(+), 19 deletions(-)

diff --git a/gfx/layers/composite/AsyncCompositionManager.cpp b/gfx/layers/composite/AsyncCompositionManager.cpp
index 5f7a9f11460..f985f37f437 100644
--- a/gfx/layers/composite/AsyncCompositionManager.cpp
+++ b/gfx/layers/composite/AsyncCompositionManager.cpp
@@ -610,6 +610,12 @@ ApplyAsyncTransformToScrollbarForContent(TimeStamp aCurrentFrame, ContainerLayer
                                          Layer* aContent, bool aScrollbarIsChild)
 {
   ContainerLayer* content = aContent->AsContainerLayer();
+
+  // We only apply the transform if the scroll-target layer has non-container
+  // children (i.e. when it has some possibly-visible content). This is to
+  // avoid moving scroll-bars in the situation that only a scroll information
+  // layer has been built for a scroll frame, as this would result in a
+  // disparity between scrollbars and visible content.
   if (!LayerHasNonContainerDescendants(content)) {
     return;
   }
@@ -683,6 +689,32 @@ ApplyAsyncTransformToScrollbarForContent(TimeStamp aCurrentFrame, ContainerLayer
   aScrollbar->AsLayerComposite()->SetShadowTransform(transform);
 }
 
+static Layer*
+FindScrolledLayerForScrollbar(ContainerLayer* aLayer, bool* aOutIsAncestor)
+{
+  // Search all siblings of aLayer and of its ancestors.
+  for (Layer* ancestor = aLayer; ancestor; ancestor = ancestor->GetParent()) {
+    for (Layer* scrollTarget = ancestor;
+         scrollTarget;
+         scrollTarget = scrollTarget->GetPrevSibling()) {
+      if (scrollTarget != aLayer &&
+          LayerIsContainerForScrollbarTarget(scrollTarget, aLayer)) {
+        *aOutIsAncestor = (scrollTarget == ancestor);
+        return scrollTarget;
+      }
+    }
+    for (Layer* scrollTarget = ancestor->GetNextSibling();
+         scrollTarget;
+         scrollTarget = scrollTarget->GetNextSibling()) {
+      if (LayerIsContainerForScrollbarTarget(scrollTarget, aLayer)) {
+        *aOutIsAncestor = false;
+        return scrollTarget;
+      }
+    }
+  }
+  return nullptr;
+}
+
 void
 AsyncCompositionManager::ApplyAsyncTransformToScrollbar(TimeStamp aCurrentFrame, ContainerLayer* aLayer)
 {
@@ -693,25 +725,11 @@ AsyncCompositionManager::ApplyAsyncTransformToScrollbar(TimeStamp aCurrentFrame,
   // Note that it is possible that the content layer is no longer there; in
   // this case we don't need to do anything because there can't be an async
   // transform on the content.
-  // We only apply the transform if the scroll-target layer has non-container
-  // children (i.e. when it has some possibly-visible content). This is to
-  // avoid moving scroll-bars in the situation that only a scroll information
-  // layer has been built for a scroll frame, as this would result in a
-  // disparity between scrollbars and visible content.
-  for (Layer* scrollTarget = aLayer->GetPrevSibling();
-       scrollTarget;
-       scrollTarget = scrollTarget->GetPrevSibling()) {
-    if (LayerIsContainerForScrollbarTarget(scrollTarget, aLayer)) {
-      // Found a sibling that matches our criteria
-      ApplyAsyncTransformToScrollbarForContent(aCurrentFrame, aLayer, scrollTarget, false);
-      return;
-    }
-  }
-
-  // If we didn't find a sibling, look for a parent
-  Layer* scrollTarget = aLayer->GetParent();
-  if (scrollTarget && LayerIsContainerForScrollbarTarget(scrollTarget, aLayer)) {
-    ApplyAsyncTransformToScrollbarForContent(aCurrentFrame, aLayer, scrollTarget, true);
+  bool isAncestor = false;
+  Layer* scrollTarget = FindScrolledLayerForScrollbar(aLayer, &isAncestor);
+  if (scrollTarget) {
+    ApplyAsyncTransformToScrollbarForContent(aCurrentFrame, aLayer, scrollTarget,
+                                             isAncestor);
   }
 }