From 68d407c357f293f64e9ce64305d81c788655d110 Mon Sep 17 00:00:00 2001
From: Chris Lord <chrislord.net@gmail.com>
Date: Fri, 13 Dec 2013 19:37:34 +0000
Subject: [PATCH] Bug 936500 - Always build layers for nsDisplayScrollLayer.
 r=tn

APZC information is associated with layers, and having those layers suddenly
disappear because they became occluded, or their displayports are off-screen
has undesired effects.
---
 layout/base/nsDisplayList.cpp | 10 ++++++++++
 layout/base/nsDisplayList.h   |  2 ++
 2 files changed, 12 insertions(+)

diff --git a/layout/base/nsDisplayList.cpp b/layout/base/nsDisplayList.cpp
index 6f9a0537017..b2999c01280 100644
--- a/layout/base/nsDisplayList.cpp
+++ b/layout/base/nsDisplayList.cpp
@@ -3498,6 +3498,16 @@ nsDisplayScrollLayer::BuildLayer(nsDisplayListBuilder* aBuilder,
   return layer.forget();
 }
 
+bool
+nsDisplayScrollLayer::ShouldBuildLayerEvenIfInvisible(nsDisplayListBuilder* aBuilder)
+{
+  if (nsLayoutUtils::GetDisplayPort(mScrolledFrame->GetContent(), nullptr)) {
+    return true;
+  }
+
+  return nsDisplayWrapList::ShouldBuildLayerEvenIfInvisible(aBuilder);
+}
+
 bool
 nsDisplayScrollLayer::ComputeVisibility(nsDisplayListBuilder* aBuilder,
                                         nsRegion* aVisibleRegion,
diff --git a/layout/base/nsDisplayList.h b/layout/base/nsDisplayList.h
index 2b28efd826c..77a5fab0311 100644
--- a/layout/base/nsDisplayList.h
+++ b/layout/base/nsDisplayList.h
@@ -2713,6 +2713,8 @@ public:
                                              LayerManager* aManager,
                                              const ContainerLayerParameters& aContainerParameters) MOZ_OVERRIDE;
 
+  virtual bool ShouldBuildLayerEvenIfInvisible(nsDisplayListBuilder* aBuilder) MOZ_OVERRIDE;
+
   virtual bool ComputeVisibility(nsDisplayListBuilder* aBuilder,
                                    nsRegion* aVisibleRegion,
                                    const nsRect& aAllowVisibleRegionExpansion) MOZ_OVERRIDE;