Bug 1373835 - Enable scroll snap for Element.scrollIntoView. r=botond

Depends on D21625

Differential Revision: https://phabricator.services.mozilla.com/D21626

--HG--
extra : moz-landing-system : lando

dom/base/Element.cpp

@@ -778,6 +778,9 @@ void Element::ScrollIntoView(const ScrollIntoViewOptions& aOptions) {
   } else if (aOptions.mBehavior == ScrollBehavior::Auto) {
     flags |= nsIPresShell::SCROLL_SMOOTH_AUTO;
   }
+  if (StaticPrefs::layout_css_scroll_snap_v1_enabled()) {
+    flags |= nsIPresShell::SCROLL_SNAP;
+  }
 
   presShell->ScrollContentIntoView(
       this, nsIPresShell::ScrollAxis(vpercent, nsIPresShell::SCROLL_ALWAYS),

layout/base/PresShell.cpp

@@ -3374,7 +3374,10 @@ static void ScrollToShowRect(nsIScrollableFrame* aFrameAsScrollable,
     if (gfxPrefs::ScrollBehaviorEnabled() && smoothScroll) {
       scrollMode = ScrollMode::eSmoothMsd;
     }
-    aFrameAsScrollable->ScrollTo(scrollPt, scrollMode, &allowedRange);
+    aFrameAsScrollable->ScrollTo(scrollPt, scrollMode, &allowedRange,
+                                 aFlags & nsIPresShell::SCROLL_SNAP
+                                     ? nsIScrollbarMediator::ENABLE_SNAP
+                                     : nsIScrollbarMediator::DISABLE_SNAP);
   }
 }
 

layout/base/nsIPresShell.h

@@ -778,7 +778,8 @@ class nsIPresShell : public nsStubDocumentObserver {
     SCROLL_OVERFLOW_HIDDEN = 0x02,
     SCROLL_NO_PARENT_FRAMES = 0x04,
     SCROLL_SMOOTH = 0x08,
-    SCROLL_SMOOTH_AUTO = 0x10
+    SCROLL_SMOOTH_AUTO = 0x10,
+    SCROLL_SNAP = 0x20
   };
   /**
    * Scrolls the view of the document so that the given area of a frame