Bug 1241371. Properly update mLastUpdateImagesPos on scrollframes when the image visibility pass encounters them. r=mats

This is a regression from bug 1002992 where we switch from the display list builder to the frame tree walker and didn't update mLastUpdateImagesPos in the frame walker.

layout/base/nsPresShell.cpp

@@ -5577,6 +5577,7 @@ PresShell::MarkImagesInSubtreeVisible(nsIFrame* aFrame, const nsRect& aRect)
 
   nsIScrollableFrame* scrollFrame = do_QueryFrame(aFrame);
   if (scrollFrame) {
+    scrollFrame->NotifyImageVisibilityUpdate();
     nsRect displayPort;
     bool usingDisplayport =
       nsLayoutUtils::GetDisplayPortForVisibilityTesting(

layout/generic/nsGfxScrollFrame.cpp

@@ -2474,6 +2474,12 @@ ScrollFrameHelper::ScheduleSyntheticMouseMove()
         ScrollActivityCallback, this, 100, nsITimer::TYPE_ONE_SHOT);
 }
 
+void
+ScrollFrameHelper::NotifyImageVisibilityUpdate()
+{
+  mLastUpdateImagesPos = GetScrollPosition();
+}
+
 void
 ScrollFrameHelper::ScrollToImpl(nsPoint aPt, const nsRect& aRange, nsIAtom* aOrigin)
 {
@@ -2896,7 +2902,7 @@ ScrollFrameHelper::BuildDisplayList(nsDisplayListBuilder*   aBuilder,
                                     const nsDisplayListSet& aLists)
 {
   if (aBuilder->IsForImageVisibility()) {
-    mLastUpdateImagesPos = GetScrollPosition();
+    NotifyImageVisibilityUpdate();
   }
 
   mOuter->DisplayBorderBackgroundOutline(aBuilder, aLists);

layout/generic/nsGfxScrollFrame.h

@@ -367,6 +367,7 @@ public:
   bool DecideScrollableLayer(nsDisplayListBuilder* aBuilder,
                              nsRect* aDirtyRect,
                              bool aAllowCreateDisplayPort);
+  void NotifyImageVisibilityUpdate();
 
   void ScheduleSyntheticMouseMove();
   static void ScrollActivityCallback(nsITimer *aTimer, void* anInstance);
@@ -849,6 +850,9 @@ public:
                                      bool aAllowCreateDisplayPort) override {
     return mHelper.DecideScrollableLayer(aBuilder, aDirtyRect, aAllowCreateDisplayPort);
   }
+  virtual void NotifyImageVisibilityUpdate() override {
+    mHelper.NotifyImageVisibilityUpdate();
+  }
 
   // nsIStatefulFrame
   NS_IMETHOD SaveState(nsPresState** aState) override {
@@ -1316,7 +1320,9 @@ public:
                                      bool aAllowCreateDisplayPort) override {
     return mHelper.DecideScrollableLayer(aBuilder, aDirtyRect, aAllowCreateDisplayPort);
   }
-
+  virtual void NotifyImageVisibilityUpdate() override {
+    mHelper.NotifyImageVisibilityUpdate();
+  }
 
 #ifdef DEBUG_FRAME_DUMP
   virtual nsresult GetFrameName(nsAString& aResult) const override;

layout/generic/nsIScrollableFrame.h

@@ -441,6 +441,11 @@ public:
   virtual bool DecideScrollableLayer(nsDisplayListBuilder* aBuilder,
                                      nsRect* aDirtyRect,
                                      bool aAllowCreateDisplayPort) = 0;
+
+  /**
+   * Notification that this scroll frame is getting its image visibility updated.
+   */
+  virtual void NotifyImageVisibilityUpdate() = 0;
 };
 
 #endif