Bug 1365830 - Don't flush layout for (0, 0) in nsGlobalWindow::ScrollTo(); r=bzbarsky

Accurate layout information for this operation isn't ever
needed.

dom/base/nsGlobalWindow.cpp

@@ -8433,7 +8433,14 @@ nsGlobalWindow::ScrollTo(double aXScroll, double aYScroll)
 void
 nsGlobalWindow::ScrollTo(const ScrollToOptions& aOptions)
 {
-  FlushPendingNotifications(FlushType::Layout);
+  // When scrolling to a non-zero offset, we need to determine whether that
+  // position is within our scrollable range, so we need updated layout
+  // information which requires a layout flush, otherwise all we need is to
+  // flush frames to be able to access our scrollable frame here.
+  FlushType flushType = (aScroll.x || aScroll.y) ?
+                          FlushType::Layout :
+                          FlushType::Frames;
+  FlushPendingNotifications(flushType);
   nsIScrollableFrame *sf = GetScrollFrame();
 
   if (sf) {
@@ -8459,7 +8466,14 @@ void
 nsGlobalWindow::ScrollTo(const CSSIntPoint& aScroll,
                          const ScrollOptions& aOptions)
 {
-  FlushPendingNotifications(FlushType::Layout);
+  // When scrolling to a non-zero offset, we need to determine whether that
+  // position is within our scrollable range, so we need updated layout
+  // information which requires a layout flush, otherwise all we need is to
+  // flush frames to be able to access our scrollable frame here.
+  FlushType flushType = (aScroll.x || aScroll.y) ?
+                          FlushType::Layout :
+                          FlushType::Frames;
+  FlushPendingNotifications(flushType);
   nsIScrollableFrame *sf = GetScrollFrame();
 
   if (sf) {