Bug 1498812 - Part 10: Return whether SetVisualViewportOffset was a no-op. r=botond

So that the caller doesn't have to retrieve and compare the previous viewport
offset himself.

Differential Revision: https://phabricator.services.mozilla.com/D15785

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

layout/base/PresShell.cpp

@@ -10091,17 +10091,20 @@ void nsIPresShell::SetVisualViewportSize(nscoord aWidth, nscoord aHeight) {
   }
 }
 
-void nsIPresShell::SetVisualViewportOffset(
+bool nsIPresShell::SetVisualViewportOffset(
     const nsPoint& aScrollOffset, const nsPoint& aPrevLayoutScrollPos) {
+  bool didChange = false;
   if (mVisualViewportOffset != aScrollOffset) {
     nsPoint prevOffset = mVisualViewportOffset;
     mVisualViewportOffset = aScrollOffset;
+    didChange = true;
 
     if (auto* window = nsGlobalWindowInner::Cast(mDocument->GetInnerWindow())) {
       window->VisualViewport()->PostScrollEvent(prevOffset,
                                                 aPrevLayoutScrollPos);
     }
   }
+  return didChange;
 }
 
 nsPoint nsIPresShell::GetVisualViewportOffsetRelativeToLayoutViewport() const {

layout/base/nsIPresShell.h

@@ -1650,7 +1650,10 @@ class nsIPresShell : public nsStubDocumentObserver {
     return mVisualViewportSize;
   }
 
-  void SetVisualViewportOffset(const nsPoint& aScrollOffset,
+  /**
+   * The return value indicates whether the offset actually changed.
+   */
+  bool SetVisualViewportOffset(const nsPoint& aScrollOffset,
                                const nsPoint& aPrevLayoutScrollPos);
 
   nsPoint GetVisualViewportOffset() const { return mVisualViewportOffset; }