Bug 1220114. Part 1. Change ScrollFrameHelper::DecideScrollableLayer to recompute the current animated geometry root any time mWillBuildScrollableLayer changes in addition to when usingDisplayPort changes. r=mstange

Changes in either one can result in AGRs being created or removed (although nothing removes them at this point in time).

layout/generic/nsGfxScrollFrame.cpp

@@ -3160,10 +3160,14 @@ ScrollFrameHelper::DecideScrollableLayer(nsDisplayListBuilder* aBuilder,
                                          nsRect* aDirtyRect,
                                          bool aAllowCreateDisplayPort)
 {
+  // Save and check if this changes so we can recompute the current agr.
+  bool oldWillBuildScrollableLayer = mWillBuildScrollableLayer;
+
+  bool wasUsingDisplayPort = false;
   bool usingDisplayPort = false;
   nsIContent* content = mOuter->GetContent();
   if (aBuilder->IsPaintingToWindow()) {
-    bool wasUsingDisplayPort = nsLayoutUtils::GetDisplayPort(content);
+    wasUsingDisplayPort = nsLayoutUtils::GetDisplayPort(content);
 
     nsRect displayportBase = *aDirtyRect;
     nsPresContext* pc = mOuter->PresContext();
@@ -3188,12 +3192,6 @@ ScrollFrameHelper::DecideScrollableLayer(nsDisplayListBuilder* aBuilder,
     // Override the dirty rectangle if the displayport has been set.
     if (usingDisplayPort) {
       *aDirtyRect = displayPort;
-
-      // The cached animated geometry root for the display builder is out of
-      // date if we just introduced a new animated geometry root.
-      if (!wasUsingDisplayPort) {
-        aBuilder->RecomputeCurrentAnimatedGeometryRoot();
-      }
     }
   }
 
@@ -3205,6 +3203,12 @@ ScrollFrameHelper::DecideScrollableLayer(nsDisplayListBuilder* aBuilder,
   // so that APZ can implement scroll grabbing.
   mWillBuildScrollableLayer = usingDisplayPort || nsContentUtils::HasScrollgrab(content);
 
+  // The cached animated geometry root for the display builder is out of
+  // date if we just introduced a new animated geometry root.
+  if ((oldWillBuildScrollableLayer != mWillBuildScrollableLayer) || (wasUsingDisplayPort != usingDisplayPort)) {
+    aBuilder->RecomputeCurrentAnimatedGeometryRoot();
+  }
+
   if (gfxPrefs::LayoutUseContainersForRootFrames() && mWillBuildScrollableLayer && mIsRoot) {
     mIsScrollableLayerInRootContainer = true;
   }