Bug 982141 - Expose the logic used to decide whether a scrollable frame should be async scrollable. r=tn

--HG--
extra : rebase_source : 6ce26d8d54bbf75c79b10fd8a8dc449b0d417387

layout/generic/nsGfxScrollFrame.cpp

@@ -1029,6 +1029,37 @@ ScrollFrameHelper::HandleScrollbarStyleSwitching()
   }
 }
 
+static bool IsFocused(nsIContent* aContent)
+{
+  // Some content elements, like the GetContent() of a scroll frame
+  // for a text input field, are inside anonymous subtrees, but the focus
+  // manager always reports a non-anonymous element as the focused one, so
+  // walk up the tree until we reach a non-anonymous element.
+  while (aContent && aContent->IsInAnonymousSubtree()) {
+    aContent = aContent->GetParent();
+  }
+
+  return aContent ? nsContentUtils::IsFocusedContent(aContent) : false;
+}
+
+bool
+ScrollFrameHelper::WantAsyncScroll() const
+{
+  nsRect scrollRange = GetScrollRange();
+  ScrollbarStyles styles = GetScrollbarStylesFromFrame();
+  bool isFocused = IsFocused(mOuter->GetContent());
+  bool isVScrollable = (scrollRange.height > 0)
+                    && (styles.mVertical != NS_STYLE_OVERFLOW_HIDDEN);
+  bool isHScrollable = (scrollRange.width > 0)
+                    && (styles.mHorizontal != NS_STYLE_OVERFLOW_HIDDEN);
+  // The check for scroll bars was added in bug 825692 to prevent layerization
+  // of text inputs for performance reasons. However, if a text input is
+  // focused we want to layerize it so we can async scroll it (bug 946408).
+  bool isVAsyncScrollable = isVScrollable && (mVScrollbarBox || isFocused);
+  bool isHAsyncScrollable = isHScrollable && (mHScrollbarBox || isFocused);
+  return isVAsyncScrollable || isHAsyncScrollable;
+}
+
 nsresult
 nsXULScrollFrame::CreateAnonymousContent(nsTArray<ContentInfo>& aElements)
 {
@@ -2306,19 +2337,6 @@ ScrollFrameHelper::ExpandRect(const nsRect& aRect) const
   return rect;
 }
 
-static bool IsFocused(nsIContent* aContent)
-{
-  // Some content elements, like the GetContent() of a scroll frame
-  // for a text input field, are inside anonymous subtrees, but the focus
-  // manager always reports a non-anonymous element as the focused one, so
-  // walk up the tree until we reach a non-anonymous element.
-  while (aContent && aContent->IsInAnonymousSubtree()) {
-    aContent = aContent->GetParent();
-  }
-
-  return aContent ? nsContentUtils::IsFocusedContent(aContent) : false;
-}
-
 static bool
 ShouldBeClippedByFrame(nsIFrame* aClipFrame, nsIFrame* aClippedFrame)
 {
@@ -2577,18 +2595,6 @@ ScrollFrameHelper::BuildDisplayList(nsDisplayListBuilder*   aBuilder,
   if (mShouldBuildScrollableLayer) {
     shouldBuildLayer = true;
   } else {
-    nsRect scrollRange = GetScrollRange();
-    ScrollbarStyles styles = GetScrollbarStylesFromFrame();
-    bool isFocused = IsFocused(mOuter->GetContent());
-    bool isVScrollable = (scrollRange.height > 0)
-                      && (styles.mVertical != NS_STYLE_OVERFLOW_HIDDEN);
-    bool isHScrollable = (scrollRange.width > 0)
-                      && (styles.mHorizontal != NS_STYLE_OVERFLOW_HIDDEN);
-    // The check for scroll bars was added in bug 825692 to prevent layerization
-    // of text inputs for performance reasons. However, if a text input is
-    // focused we want to layerize it so we can async scroll it (bug 946408).
-    bool wantLayerV = isVScrollable && (mVScrollbarBox || isFocused);
-    bool wantLayerH = isHScrollable && (mHScrollbarBox || isFocused);
     // TODO Turn this on for inprocess OMTC on all platforms
     bool wantSubAPZC = gfxPrefs::APZSubframeEnabled();
 #ifdef MOZ_WIDGET_GONK
@@ -2598,7 +2604,7 @@ ScrollFrameHelper::BuildDisplayList(nsDisplayListBuilder*   aBuilder,
 #endif
     shouldBuildLayer =
       wantSubAPZC &&
-      (wantLayerV || wantLayerH) &&
+      WantAsyncScroll() &&
       // If we are the root scroll frame for the display root then we don't need a scroll
       // info layer to make a RecordFrameMetrics call for us as
       // nsDisplayList::PaintForFrame already calls RecordFrameMetrics for us.

layout/generic/nsGfxScrollFrame.h

@@ -308,6 +308,7 @@ public:
       mOriginOfLastScroll = nullptr;
     }
   }
+  bool WantAsyncScroll() const;
 
   // owning references to the nsIAnonymousContentCreator-built content
   nsCOMPtr<nsIContent> mHScrollbarContent;
@@ -661,6 +662,9 @@ public:
   virtual void ResetOriginIfScrollAtGeneration(uint32_t aGeneration) MOZ_OVERRIDE {
     mHelper.ResetOriginIfScrollAtGeneration(aGeneration);
   }
+  virtual bool WantAsyncScroll() const MOZ_OVERRIDE {
+    return mHelper.WantAsyncScroll();
+  }
 
   // nsIStatefulFrame
   NS_IMETHOD SaveState(nsPresState** aState) MOZ_OVERRIDE {
@@ -965,6 +969,9 @@ public:
   virtual void ResetOriginIfScrollAtGeneration(uint32_t aGeneration) MOZ_OVERRIDE {
     mHelper.ResetOriginIfScrollAtGeneration(aGeneration);
   }
+  virtual bool WantAsyncScroll() const MOZ_OVERRIDE {
+    return mHelper.WantAsyncScroll();
+  }
 
   // nsIStatefulFrame
   NS_IMETHOD SaveState(nsPresState** aState) MOZ_OVERRIDE {

layout/generic/nsIScrollableFrame.h

@@ -291,6 +291,11 @@ public:
    * counter.
    */
   virtual void ResetOriginIfScrollAtGeneration(uint32_t aGeneration) = 0;
+  /**
+   * Determine whether it is desirable to be able to asynchronously scroll this
+   * scroll frame.
+   */
+  virtual bool WantAsyncScroll() const = 0;
 };
 
 #endif