Bug 1725569. Send visualViewport resize events for iframes. r=botond

Differential Revision: https://phabricator.services.mozilla.com/D127941

Author: tnikkel

layout/base/PresShell.h

@@ -2967,6 +2967,7 @@ class PresShell final : public nsStubDocumentObserver,
   // needed for the synthetic mouse events.
   layers::ScrollableLayerGuid mMouseEventTargetGuid;
 
+  // Only populated on root content documents.
   nsSize mVisualViewportSize;
 
   // The focus information needed for async keyboard scrolling

layout/generic/nsGfxScrollFrame.cpp

@@ -1371,6 +1371,17 @@ void nsHTMLScrollFrame::Reflow(nsPresContext* aPresContext,
       // visual viewport is updated to account for that before we read the
       // visual viewport size.
       manager->UpdateVisualViewportSizeForPotentialScrollbarChange();
+    } else if ((oldScrollAreaBounds.Size() != newScrollAreaBounds.Size()) &&
+               !HasAnyStateBits(NS_FRAME_FIRST_REFLOW)) {
+      // We want to make sure to send a visual viewport resize event if the
+      // scrollport changed sizes for root scroll frames. The
+      // MobileViewportManager will do that, but if we don't have one (ie we
+      // aren't a root content document for example) we have to send one
+      // ourselves.
+      if (auto* window = nsGlobalWindowInner::Cast(
+              aPresContext->Document()->GetInnerWindow())) {
+        window->VisualViewport()->PostResizeEvent();
+      }
     }
   }