From e41f4e8c1afa92b5f0dc87ddc6b3b7e8c42e6aa1 Mon Sep 17 00:00:00 2001
From: Markus Stange <mstange@themasta.com>
Date: Fri, 11 Mar 2016 16:09:16 -0500
Subject: [PATCH] Bug 1255068 - Do not allow empty transaction transform
 changes if the scroll position has changed since the last paint. r=kats,
 r=mattwoodrow

MozReview-Commit-ID: 7exuhuf3HTJ

--HG--
extra : rebase_source : b131cdc68e07ab8dfdc6ae56caa24cc0ced5102e
---
 layout/generic/nsFrame.cpp | 39 ++++++++++++++++++++++++++++++++++++++
 1 file changed, 39 insertions(+)

diff --git a/layout/generic/nsFrame.cpp b/layout/generic/nsFrame.cpp
index 5b7ca31d42ae..b85ded0fbc49 100644
--- a/layout/generic/nsFrame.cpp
+++ b/layout/generic/nsFrame.cpp
@@ -5272,6 +5272,37 @@ nsIFrame::InvalidateFrameWithRect(const nsRect& aRect, uint32_t aDisplayItemKey)
 
 /*static*/ uint8_t nsIFrame::sLayerIsPrerenderedDataKey;
 
+static bool
+DoesLayerHaveOutOfDateFrameMetrics(Layer* aLayer)
+{
+  for (uint32_t i = 0; i < aLayer->GetFrameMetricsCount(); i++) {
+    const FrameMetrics& metrics = aLayer->GetFrameMetrics(i);
+    nsIScrollableFrame* scrollableFrame =
+      nsLayoutUtils::FindScrollableFrameFor(metrics.GetScrollId());
+    if (!scrollableFrame) {
+      // This shouldn't happen, so let's do the safe thing and trigger a full
+      // paint if it does.
+      return true;
+    }
+    nsPoint scrollPosition = scrollableFrame->GetScrollPosition();
+    if (metrics.GetScrollOffset() != CSSPoint::FromAppUnits(scrollPosition)) {
+      return true;
+    }
+  }
+  return false;
+}
+
+static bool
+DoesLayerOrAncestorsHaveOutOfDateFrameMetrics(Layer* aLayer)
+{
+  for (Layer* layer = aLayer; layer; layer = layer->GetParent()) {
+    if (DoesLayerHaveOutOfDateFrameMetrics(layer)) {
+      return true;
+    }
+  }
+  return false;
+}
+
 bool
 nsIFrame::TryUpdateTransformOnly(Layer** aLayerResult)
 {
@@ -5284,6 +5315,14 @@ nsIFrame::TryUpdateTransformOnly(Layer** aLayerResult)
     return false;
   }
 
+  if (DoesLayerOrAncestorsHaveOutOfDateFrameMetrics(layer)) {
+    // At least one scroll frame that can affect the position of this layer
+    // has changed its scroll offset since the last paint. Schedule a full
+    // paint to make sure that this layer's transform and all the frame
+    // metrics that affect it are in sync.
+    return false;
+  }
+
   gfx::Matrix4x4 transform3d;
   if (!nsLayoutUtils::GetLayerTransformForFrame(this, &transform3d)) {
     // We're not able to compute a layer transform that we know would