Bug 945584: Part 8 - Implement Scroll Snapping for Middle Mouse Button Scrolls (v2 Patch), r=roc, r=bz

- Implemented scroll snapping at the end of a middle-mouse-button scroll.
- As this scrolling occurs within chrome Javascript, chrome-only DOM methods
  had to be added: window.MozScrollSnap and element.MozScrollSnap
- Bug 1137937 tracks implementation of a replacement for these chome-only DOM methods,
  to be replaced with a web accessible API.

--HG--
extra : rebase_source : f511296dbdbfd97e3da391d2a0b67fd3f6edca6b

dom/base/Element.cpp

@@ -797,6 +797,15 @@ Element::ScrollByNoFlush(int32_t aDx, int32_t aDy)
   return (before != after);
 }
 
+void
+Element::MozScrollSnap()
+{
+  nsIScrollableFrame* sf = GetScrollFrame(nullptr, false);
+  if (sf) {
+    sf->ScrollSnap();
+  }
+}
+
 static nsSize GetScrollRectSizeForOverflowVisibleFrame(nsIFrame* aFrame)
 {
   if (!aFrame) {

dom/base/Element.h

@@ -762,6 +762,7 @@ public:
   void SetScrollLeft(int32_t aScrollLeft);
   int32_t ScrollWidth();
   int32_t ScrollHeight();
+  void MozScrollSnap();
   int32_t ClientTop()
   {
     return nsPresContext::AppUnitsToIntCSSPixels(GetClientAreaRect().y);

dom/base/nsGlobalWindow.cpp

@@ -7355,6 +7355,16 @@ nsGlobalWindow::ScrollByPages(int32_t numPages,
   }
 }
 
+void
+nsGlobalWindow::MozScrollSnap()
+{
+  FlushPendingNotifications(Flush_Layout);
+  nsIScrollableFrame *sf = GetScrollFrame();
+  if (sf) {
+    sf->ScrollSnap();
+  }
+}
+
 void
 nsGlobalWindow::MozRequestOverfill(OverfillCallback& aCallback,
                                    mozilla::ErrorResult& aError)

dom/base/nsGlobalWindow.h

@@ -946,6 +946,7 @@ public:
                      const mozilla::dom::ScrollOptions& aOptions);
   void ScrollByPages(int32_t numPages,
                      const mozilla::dom::ScrollOptions& aOptions);
+  void MozScrollSnap();
   int32_t GetInnerWidth(mozilla::ErrorResult& aError);
   void SetInnerWidth(int32_t aInnerWidth, mozilla::ErrorResult& aError);
   int32_t GetInnerHeight(mozilla::ErrorResult& aError);

dom/webidl/Element.webidl

@@ -185,6 +185,11 @@ partial interface Element {
   void scrollTo(optional ScrollToOptions options);
   void scrollBy(unrestricted double x, unrestricted double y);
   void scrollBy(optional ScrollToOptions options);
+  // mozScrollSnap is used by chrome to perform scroll snapping after the
+  // user performs actions that may affect scroll position
+  // mozScrollSnap is deprecated, to be replaced by a web accessible API, such
+  // as an extension to the ScrollOptions dictionary.  See bug 1137937.
+  [ChromeOnly] void mozScrollSnap();
 
   readonly attribute long clientTop;
   readonly attribute long clientLeft;

dom/webidl/Window.webidl

@@ -190,6 +190,11 @@ partial interface Window {
   void scrollTo(optional ScrollToOptions options);
   void scrollBy(unrestricted double x, unrestricted double y);
   void scrollBy(optional ScrollToOptions options);
+  // mozScrollSnap is used by chrome to perform scroll snapping after the
+  // user performs actions that may affect scroll position
+  // mozScrollSnap is deprecated, to be replaced by a web accessible API, such
+  // as an extension to the ScrollOptions dictionary.  See bug 1137937.
+  [ChromeOnly] void mozScrollSnap();
   [Replaceable, Throws] readonly attribute long scrollX;
   [Throws] readonly attribute long pageXOffset;
   [Replaceable, Throws] readonly attribute long scrollY;

toolkit/content/browser-content.js

@@ -146,6 +146,7 @@ let ClickEventHandler = {
 
   stopScroll: function() {
     if (this._scrollable) {
+      this._scrollable.mozScrollSnap();
       this._scrollable = null;
 
       Cc["@mozilla.org/eventlistenerservice;1"]