Bug 868498: Enable switching between overlay and regular scrollbars without browser restart on Mac OSX. r=roc

layout/generic/nsGfxScrollFrame.cpp

@@ -726,7 +726,6 @@ GetBrowserRoot(nsIContent* aContent)
   return nullptr;
 }
 
-
 NS_IMETHODIMP
 nsHTMLScrollFrame::Reflow(nsPresContext*           aPresContext,
                           nsHTMLReflowMetrics&     aDesiredSize,
@@ -736,6 +735,8 @@ nsHTMLScrollFrame::Reflow(nsPresContext*           aPresContext,
   DO_GLOBAL_REFLOW_COUNT("nsHTMLScrollFrame");
   DISPLAY_REFLOW(aPresContext, this, aReflowState, aDesiredSize, aStatus);
 
+  mInner.HandleScrollbarStyleSwitching();
+
   ScrollReflowState state(this, aReflowState);
   // sanity check: ensure that if we have no scrollbar, we treat it
   // as hidden.
@@ -774,10 +775,6 @@ nsHTMLScrollFrame::Reflow(nsPresContext*           aPresContext,
   nsRect oldScrollAreaBounds = mInner.mScrollPort;
   nsRect oldScrolledAreaBounds =
     mInner.mScrolledFrame->GetScrollableOverflowRectRelativeToParent();
-  // Adjust to a multiple of device pixels to restore the invariant that
-  // oldScrollPosition is a multiple of device pixels. This could have been
-  // thrown out by a zoom change.
-  nsIntPoint ptDevPx;
   nsPoint oldScrollPosition = mInner.GetScrollPosition();
 
   state.mComputedBorder = aReflowState.mComputedBorderPadding -
@@ -971,6 +968,23 @@ nsGfxScrollFrameInner::GetNondisappearingScrollbarWidth(nsBoxLayoutState* aState
   return GetDesiredScrollbarSizes(aState).LeftRight();
 }
 
+void
+nsGfxScrollFrameInner::HandleScrollbarStyleSwitching()
+{
+  // Check if we switched between scrollbar styles.
+  if (mScrollbarActivity &&
+      LookAndFeel::GetInt(LookAndFeel::eIntID_UseOverlayScrollbars) == 0) {
+    mScrollbarActivity->Destroy();
+    mScrollbarActivity = nullptr;
+    mOuter->PresContext()->ThemeChanged();
+  }
+  else if (!mScrollbarActivity &&
+           LookAndFeel::GetInt(LookAndFeel::eIntID_UseOverlayScrollbars) != 0) {
+    mScrollbarActivity = new ScrollbarActivity(do_QueryFrame(mOuter));
+    mOuter->PresContext()->ThemeChanged();
+  }
+}
+
 nsresult
 nsXULScrollFrame::CreateAnonymousContent(nsTArray<ContentInfo>& aElements)
 {

layout/generic/nsGfxScrollFrame.h

@@ -269,6 +269,8 @@ public:
   void ScheduleSyntheticMouseMove();
   static void ScrollActivityCallback(nsITimer *aTimer, void* anInstance);
 
+  void HandleScrollbarStyleSwitching();
+
   // owning references to the nsIAnonymousContentCreator-built content
   nsCOMPtr<nsIContent> mHScrollbarContent;
   nsCOMPtr<nsIContent> mVScrollbarContent;

widget/cocoa/nsChildView.mm

@@ -2808,7 +2808,7 @@ NSEvent* gLastDragMouseDownEvent = nil;
                                              object:nil];
   // TODO: replace the string with the constant once we build with the 10.7 SDK
   [[NSNotificationCenter defaultCenter] addObserver:self
-                                           selector:@selector(systemMetricsChanged)
+                                           selector:@selector(scrollbarSystemMetricChanged)
                                                name:@"NSPreferredScrollerStyleDidChangeNotification"
                                              object:nil];
   [[NSDistributedNotificationCenter defaultCenter] addObserver:self
@@ -2961,6 +2961,18 @@ NSEvent* gLastDragMouseDownEvent = nil;
     mGeckoChild->NotifyThemeChanged();
 }
 
+- (void)scrollbarSystemMetricChanged
+{
+  [self systemMetricsChanged];
+
+  if (mGeckoChild) {
+    nsIWidgetListener* listener = mGeckoChild->GetWidgetListener();
+    if (listener) {
+      listener->GetPresShell()->ReconstructFrames();
+    }
+  }
+}
+
 - (void)setNeedsPendingDisplay
 {
   NS_OBJC_BEGIN_TRY_ABORT_BLOCK;