Bug 1530253 - Trigger re-snap on reflows and in RecomputePosition. r=botond,emilio

The basic machinery to trigger re-snap is same as what scroll anchoring does,
queueing a triggering request and flush the queued requests after reflows
finished.

Depends on D148862

Differential Revision: https://phabricator.services.mozilla.com/D148863

layout/base/PresShell.cpp

@@ -201,6 +201,7 @@
 #include "nsIDocShellTreeOwner.h"
 #include "nsClassHashtable.h"
 #include "nsHashKeys.h"
+#include "ScrollSnap.h"
 #include "VisualViewport.h"
 #include "ZoomConstraintsClient.h"
 
@@ -1363,6 +1364,7 @@ void PresShell::Destroy() {
   mFramesToDirty.Clear();
   mPendingScrollAnchorSelection.Clear();
   mPendingScrollAnchorAdjustment.Clear();
+  mPendingScrollResnap.Clear();
 
   if (mViewManager) {
     // Clear the view manager's weak pointer back to |this| in case it
@@ -2248,6 +2250,7 @@ void PresShell::NotifyDestroyingFrame(nsIFrame* aFrame) {
     if (scrollableFrame) {
       mPendingScrollAnchorSelection.Remove(scrollableFrame);
       mPendingScrollAnchorAdjustment.Remove(scrollableFrame);
+      mPendingScrollResnap.Remove(scrollableFrame);
     }
   }
 }
@@ -2687,6 +2690,17 @@ void PresShell::FlushPendingScrollAnchorAdjustments() {
   mPendingScrollAnchorAdjustment.Clear();
 }
 
+void PresShell::PostPendingScrollResnap(nsIScrollableFrame* aScrollableFrame) {
+  mPendingScrollResnap.Insert(aScrollableFrame);
+}
+
+void PresShell::FlushPendingScrollResnap() {
+  for (nsIScrollableFrame* scrollableFrame : mPendingScrollResnap) {
+    scrollableFrame->TryResnap();
+  }
+  mPendingScrollResnap.Clear();
+}
+
 void PresShell::FrameNeedsReflow(nsIFrame* aFrame,
                                  IntrinsicDirty aIntrinsicDirty,
                                  nsFrameState aBitToAdd,
@@ -4367,6 +4381,8 @@ void PresShell::DoFlushPendingNotifications(mozilla::ChangesToFlush aFlush) {
       }
     }
 
+    FlushPendingScrollResnap();
+
     if (flushType >= FlushType::Layout) {
       if (!mIsDestroying) {
         viewManager->UpdateWidgetGeometry();