Bug 1195431 - Extract a couple of helper functions. r=tn

--HG--
extra : commitid : FK958utOKA5

layout/base/nsLayoutUtils.cpp

@@ -3019,6 +3019,25 @@ nsLayoutUtils::CalculateAndSetDisplayPortMargins(nsIScrollableFrame* aScrollFram
       content, presShell, displayportMargins, 0, aRepaintMode);
 }
 
+bool
+nsLayoutUtils::WantDisplayPort(const nsDisplayListBuilder* aBuilder,
+                               nsIFrame* aScrollFrame)
+{
+  nsIScrollableFrame* scrollableFrame = do_QueryFrame(aScrollFrame);
+  if (!scrollableFrame) {
+    return false;
+  }
+
+  // We perform an optimization where we ensure that at least one
+  // async-scrollable frame (i.e. one that WantAsyncScroll()) has a displayport.
+  // If that's not the case yet, and we are async-scrollable, we will get a
+  // displayport.
+  return aBuilder->IsPaintingToWindow() &&
+         nsLayoutUtils::AsyncPanZoomEnabled(aScrollFrame) &&
+         !aBuilder->HaveScrollableDisplayPort() &&
+         scrollableFrame->WantAsyncScroll();
+}
+
 bool
 nsLayoutUtils::GetOrMaybeCreateDisplayPort(nsDisplayListBuilder& aBuilder,
                                            nsIFrame* aScrollFrame,
@@ -3037,17 +3056,7 @@ nsLayoutUtils::GetOrMaybeCreateDisplayPort(nsDisplayListBuilder& aBuilder,
 
   bool haveDisplayPort = GetDisplayPort(content, aOutDisplayport);
 
-  // We perform an optimization where we ensure that at least one
-  // async-scrollable frame (i.e. one that WantsAsyncScroll()) has a displayport.
-  // If that's not the case yet, and we are async-scrollable, we will get a
-  // displayport.
-  // Note: we only do this in processes where we do subframe scrolling to
-  //       begin with (i.e., not in the parent process on B2G).
-  if (aBuilder.IsPaintingToWindow() &&
-      nsLayoutUtils::AsyncPanZoomEnabled(aScrollFrame) &&
-      !aBuilder.HaveScrollableDisplayPort() &&
-      scrollableFrame->WantAsyncScroll()) {
-
+  if (WantDisplayPort(&aBuilder, aScrollFrame)) {
     // If we don't already have a displayport, calculate and set one.
     if (!haveDisplayPort) {
       CalculateAndSetDisplayPortMargins(scrollableFrame, nsLayoutUtils::RepaintMode::DoNotRepaint);

layout/base/nsLayoutUtils.h

@@ -2654,6 +2654,12 @@ public:
   static bool CalculateAndSetDisplayPortMargins(nsIScrollableFrame* aScrollFrame,
                                                 RepaintMode aRepaintMode);
 
+  /**
+   * Return true if GetOrMaybeCreateDisplayPort would create a displayport.
+   */
+  static bool WantDisplayPort(const nsDisplayListBuilder* aBuilder,
+                              nsIFrame* aScrollFrame);
+
   /**
    * Get the display port for |aScrollFrame|'s content. If |aScrollFrame|
    * WantsAsyncScroll() and we don't have a scrollable displayport yet (as

layout/generic/nsGfxScrollFrame.cpp

@@ -2778,6 +2778,13 @@ ClipListsExceptCaret(nsDisplayListCollection* aLists,
   ClipItemsExceptCaret(aLists->Content(), aBuilder, aClipFrame, clip, aUsingDisplayPort);
 }
 
+bool
+ScrollFrameHelper::IsUsingDisplayPort(const nsDisplayListBuilder* aBuilder) const
+{
+  return aBuilder->IsPaintingToWindow() &&
+    nsLayoutUtils::GetDisplayPort(mOuter->GetContent());
+}
+
 void
 ScrollFrameHelper::BuildDisplayList(nsDisplayListBuilder*   aBuilder,
                                     const nsRect&           aDirtyRect,
@@ -2818,8 +2825,7 @@ ScrollFrameHelper::BuildDisplayList(nsDisplayListBuilder*   aBuilder,
   bool createLayersForScrollbars = mIsRoot &&
     mOuter->PresContext()->IsRootContentDocument();
 
-  bool usingDisplayPort = aBuilder->IsPaintingToWindow() &&
-    nsLayoutUtils::GetDisplayPort(mOuter->GetContent());
+  bool usingDisplayPort = IsUsingDisplayPort(aBuilder);
 
   if (aBuilder->GetIgnoreScrollFrame() == mOuter || IsIgnoringViewportClipping()) {
     // Root scrollframes have FrameMetrics and clipping on their container

layout/generic/nsGfxScrollFrame.h

@@ -81,6 +81,8 @@ public:
                            bool                    aCreateLayer,
                            bool                    aPositioned);
 
+  bool IsUsingDisplayPort(const nsDisplayListBuilder* aBuilder) const;
+
   bool GetBorderRadii(const nsSize& aFrameSize, const nsSize& aBorderArea,
                       Sides aSkipSides, nscoord aRadii[8]) const;