Bug 1501665 Part 7: Add a new function to allow visual viewport size to be un-set. r=smaug

Once nsIPresShell::SetVisualViewportSize is called, we currently have
no way to restore the default viewport sizing behavior. This corrects that.
We need the default behavior to get correctly-placed scrollbars when
turning off meta viewport handling in Responsive Design Mode panes.

Differential Revision: https://phabricator.services.mozilla.com/D20593

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

layout/base/PresShell.cpp

@@ -10627,15 +10627,8 @@ void nsIPresShell::MarkFixedFramesForReflow(IntrinsicDirty aIntrinsicDirty) {
   }
 }
 
-void nsIPresShell::SetVisualViewportSize(nscoord aWidth, nscoord aHeight) {
-  if (!mVisualViewportSizeSet || mVisualViewportSize.width != aWidth ||
-      mVisualViewportSize.height != aHeight) {
-    mVisualViewportSizeSet = true;
-    mVisualViewportSize.width = aWidth;
-    mVisualViewportSize.height = aHeight;
-
-    if (nsIScrollableFrame* rootScrollFrame =
-            GetRootScrollFrameAsScrollable()) {
+void nsIPresShell::CompleteChangeToVisualViewportSize() {
+  if (nsIScrollableFrame* rootScrollFrame = GetRootScrollFrameAsScrollable()) {
     rootScrollFrame->MarkScrollbarsDirtyForReflow();
   }
   MarkFixedFramesForReflow(nsIPresShell::eResize);
@@ -10644,12 +10637,31 @@ void nsIPresShell::SetVisualViewportSize(nscoord aWidth, nscoord aHeight) {
     window->VisualViewport()->PostResizeEvent();
   }
 
-    if (nsIScrollableFrame* rootScrollFrame =
-            GetRootScrollFrameAsScrollable()) {
+  if (nsIScrollableFrame* rootScrollFrame = GetRootScrollFrameAsScrollable()) {
     ScrollAnchorContainer* container = rootScrollFrame->Anchor();
     container->UserScrolled();
   }
 }
+
+void nsIPresShell::SetVisualViewportSize(nscoord aWidth, nscoord aHeight) {
+  if (!mVisualViewportSizeSet || mVisualViewportSize.width != aWidth ||
+      mVisualViewportSize.height != aHeight) {
+    mVisualViewportSizeSet = true;
+    mVisualViewportSize.width = aWidth;
+    mVisualViewportSize.height = aHeight;
+
+    CompleteChangeToVisualViewportSize();
+  }
+}
+
+void nsIPresShell::ResetVisualViewportSize() {
+  if (mVisualViewportSizeSet) {
+    mVisualViewportSizeSet = false;
+    mVisualViewportSize.width = 0;
+    mVisualViewportSize.height = 0;
+
+    CompleteChangeToVisualViewportSize();
+  }
 }
 
 bool nsIPresShell::SetVisualViewportOffset(

layout/base/nsIPresShell.h

@@ -1654,6 +1654,7 @@ class nsIPresShell : public nsStubDocumentObserver {
   static void ClearMouseCapture(nsIFrame* aFrame);
 
   void SetVisualViewportSize(nscoord aWidth, nscoord aHeight);
+  void ResetVisualViewportSize();
   bool IsVisualViewportSizeSet() { return mVisualViewportSizeSet; }
   nsSize GetVisualViewportSize() {
     NS_ASSERTION(mVisualViewportSizeSet,
@@ -1661,6 +1662,10 @@ class nsIPresShell : public nsStubDocumentObserver {
     return mVisualViewportSize;
   }
 
+  // This function handles all the work after VisualViewportSize is set
+  // or reset.
+  void CompleteChangeToVisualViewportSize();
+
   /**
    * The return value indicates whether the offset actually changed.
    */