Bug 1590551. Allow nsPresContext::GetNearestWidget to work when there is no frame tree. r=botond

Differential Revision: https://phabricator.services.mozilla.com/D50135

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

layout/base/PresShell.cpp

@@ -10506,15 +10506,13 @@ RefPtr<MobileViewportManager> PresShell::GetMobileViewportManager() const {
 bool UseMobileViewportManager(PresShell* aPresShell, Document* aDocument) {
   // If we're not using APZ, we won't be able to zoom, so there is no
   // point in having an MVM.
-  if (nsViewManager* viewManager = aPresShell->GetViewManager()) {
-    if (nsView* view = viewManager->GetRootView()) {
-      if (nsIWidget* widget = view->GetNearestWidget(nullptr)) {
+  if (nsPresContext* presContext = aPresShell->GetPresContext()) {
+    if (nsIWidget* widget = presContext->GetNearestWidget()) {
       if (!widget->AsyncPanZoomEnabled()) {
         return false;
       }
     }
   }
-  }
   return nsLayoutUtils::ShouldHandleMetaViewport(aDocument) ||
          nsLayoutUtils::AllowZoomingForDocument(aDocument);
 }

layout/base/nsPresContext.cpp

@@ -817,9 +817,11 @@ nsPresContext* nsPresContext::GetToplevelContentDocumentPresContext() {
 
 nsIWidget* nsPresContext::GetNearestWidget(nsPoint* aOffset) {
   NS_ENSURE_TRUE(mPresShell, nullptr);
-  nsIFrame* frame = mPresShell->GetRootFrame();
-  NS_ENSURE_TRUE(frame, nullptr);
-  return frame->GetView()->GetNearestWidget(aOffset);
+  nsViewManager* vm = mPresShell->GetViewManager();
+  NS_ENSURE_TRUE(vm, nullptr);
+  nsView* rootView = vm->GetRootView();
+  NS_ENSURE_TRUE(rootView, nullptr);
+  return rootView->GetNearestWidget(aOffset);
 }
 
 nsIWidget* nsPresContext::GetRootWidget() const {

layout/base/nsPresContext.h

@@ -192,7 +192,7 @@ class nsPresContext : public nsISupports,
   nsPresContext* GetToplevelContentDocumentPresContext();
 
   /**
-   * Returns the nearest widget for the root frame of this.
+   * Returns the nearest widget for the root frame or view of this.
    *
    * @param aOffset     If non-null the offset from the origin of the root
    *                    frame's view to the widget's origin (usually positive)