Bug 1814668 - Fix viewport size invalidation on MVM changes. r=hiro

The MVM doesn't change the document viewer bounds nor the view manager
bounds (maybe it used to?). Right now when exiting RDM, depending on the
order of operations (if MVM reset is the last thing to happen) we end up
with a wrong viewport size, because nsDocShell::SetSize is
short-circuited here[1].

Instead, force a resize reflow with the current dimensions which is what
the dynamic toolbar code also does, for example.

[1]: https://searchfox.org/mozilla-central/rev/5ccb73c0217d1710b10d6e6e297cf3396d10ec23/view/nsViewManager.cpp#172

Differential Revision: https://phabricator.services.mozilla.com/D169806

docshell/base/nsIDocShell.idl

@@ -718,7 +718,7 @@ interface nsIDocShell : nsIDocShellTreeItem
    * <meta name="viewport"> tag is respected in a specific docshell.
    * Possible values are listed above.
    */
-  [infallible] attribute nsIDocShell_MetaViewportOverride metaViewportOverride;
+  [infallible, setter_can_run_script] attribute nsIDocShell_MetaViewportOverride metaViewportOverride;
 
   /**
    * Attribute that determines whether tracking protection is enabled.

layout/base/PresShell.cpp

@@ -1947,6 +1947,13 @@ void PresShell::RefreshZoomConstraintsForScreenSizeChange() {
   }
 }
 
+void PresShell::ForceResizeReflowWithCurrentDimensions() {
+  nscoord currentWidth = 0;
+  nscoord currentHeight = 0;
+  mViewManager->GetWindowDimensions(&currentWidth, &currentHeight);
+  ResizeReflow(currentWidth, currentHeight);
+}
+
 void PresShell::ResizeReflow(nscoord aWidth, nscoord aHeight,
                              ResizeReflowOptions aOptions) {
   if (mZoomConstraintsClient) {
@@ -11071,15 +11078,8 @@ void PresShell::MaybeRecreateMobileViewportManager(bool aAfterInitialization) {
                             ResolutionChangeOrigin::MainThreadRestore);
 
     if (aAfterInitialization) {
-      // Force a reflow to our correct size by going back to the docShell
-      // and asking it to reassert its size. This is necessary because
-      // everything underneath the docShell, like the ViewManager, has been
-      // altered by the MobileViewportManager in an irreversible way.
-      nsDocShell* docShell =
-          static_cast<nsDocShell*>(GetPresContext()->GetDocShell());
-      int32_t width, height;
-      docShell->GetSize(&width, &height);
-      docShell->SetSize(width, height, false);
+      // Force a reflow to our correct view manager size.
+      ForceResizeReflowWithCurrentDimensions();
     }
   }
 

layout/base/PresShell.h

@@ -241,7 +241,7 @@ class PresShell final : public nsStubDocumentObserver,
     return mLastOverWindowPointerLocation;
   }
 
-  void Init(nsPresContext*, nsViewManager*);
+  MOZ_CAN_RUN_SCRIPT void Init(nsPresContext*, nsViewManager*);
 
   /**
    * All callers are responsible for calling |Destroy| after calling
@@ -355,6 +355,7 @@ class PresShell final : public nsStubDocumentObserver,
   MOZ_CAN_RUN_SCRIPT bool ResizeReflowIgnoreOverride(
       nscoord aWidth, nscoord aHeight,
       ResizeReflowOptions = ResizeReflowOptions::NoOption);
+  MOZ_CAN_RUN_SCRIPT void ForceResizeReflowWithCurrentDimensions();
 
   /**
    * Add this pres shell to the refresh driver to be observed for resize
@@ -396,7 +397,8 @@ class PresShell final : public nsStubDocumentObserver,
    * Note that the assumptions that determine whether we need a mobile viewport
    * manager may have changed.
    */
-  void MaybeRecreateMobileViewportManager(bool aAfterInitialization);
+  MOZ_CAN_RUN_SCRIPT void MaybeRecreateMobileViewportManager(
+      bool aAfterInitialization);
 
   /**
    * Returns true if this document uses mobile viewport sizing (including

layout/base/nsPresContext.cpp

@@ -2876,10 +2876,7 @@ void nsPresContext::SetDynamicToolbarMaxHeight(ScreenIntCoord aHeight) {
     // PresShell::ResizeReflow ends up subtracting the new dynamic toolbar
     // height from the window dimensions and kick a reflow with the proper ICB
     // size.
-    nscoord currentWidth, currentHeight;
-    presShell->GetViewManager()->GetWindowDimensions(&currentWidth,
-                                                     &currentHeight);
-    presShell->ResizeReflow(currentWidth, currentHeight);
+    presShell->ForceResizeReflowWithCurrentDimensions();
   }
 }