Perform layout when viewport size is queried.

third_party/WebKit/Source/core/frame/LocalDOMWindow.cpp

 /*
  * Copyright (C) 2006, 2007, 2008, 2010 Apple Inc. All rights reserved.
  * Copyright (C) 2010 Nokia Corporation and/or its subsidiary(-ies)
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
@@ skipping 65 matching lines @@
 #include "core/page/Page.h"
 #include "core/page/WindowFeatures.h"
 #include "core/page/scrolling/ScrollingCoordinator.h"
 #include "core/probe/CoreProbes.h"
 #include "core/timing/DOMWindowPerformance.h"
 #include "core/timing/Performance.h"
 #include "platform/EventDispatchForbiddenScope.h"
 #include "platform/Histogram.h"
 #include "platform/WebFrameScheduler.h"
 #include "platform/loader/fetch/ResourceFetcher.h"
+#include "platform/scroll/ScrollbarTheme.h"
 #include "platform/weborigin/SecurityOrigin.h"
 #include "platform/weborigin/Suborigin.h"
 #include "public/platform/Platform.h"
 #include "public/platform/WebScreenInfo.h"
 #include "public/platform/site_engagement.mojom-blink.h"
 
 #include <memory>
 
 namespace blink {
 
@@ skipping 872 matching lines @@
     return FloatSize();
 
   FrameView* view = GetFrame()->View();
   if (!view)
     return FloatSize();
 
   Page* page = GetFrame()->GetPage();
   if (!page)
     return FloatSize();
 
-  // The main frame's viewport size depends on the page scale. Since the
-  // initial page scale depends on the content width and is set after a
-  // layout, perform one now so queries during page load will use the up to
-  // date viewport.
-  if (page->GetSettings().GetViewportEnabled() && GetFrame()->IsMainFrame())
+  // The main frame's viewport size depends on the page scale. If viewport is
+  // enabled, the initial page scale depends on the content width and is set
+  // after a layout, perform one now so queries during page load will use the
+  // up to date viewport.
+  bool affectedByScale =
+      page->GetSettings().GetViewportEnabled() && GetFrame()->IsMainFrame();
+  bool affectedByScrollbars =
+      scrollbar_inclusion == kExcludeScrollbars &&
+      !ScrollbarTheme::GetTheme().UsesOverlayScrollbars();
+
+  if (affectedByScale || affectedByScrollbars)
     document()->UpdateStyleAndLayoutIgnorePendingStylesheets();
 
   // FIXME: This is potentially too much work. We really only need to know the
   // dimensions of the parent frame's layoutObject.
   if (Frame* parent = GetFrame()->Tree().Parent()) {
     if (parent && parent->IsLocalFrame())
       ToLocalFrame(parent)
           ->GetDocument()
           ->UpdateStyleAndLayoutIgnorePendingStylesheets();
   }
@@ skipping 691 matching lines @@
   DOMWindow::Trace(visitor);
   Supplementable<LocalDOMWindow>::Trace(visitor);
 }
 
 DEFINE_TRACE_WRAPPERS(LocalDOMWindow) {
   visitor->TraceWrappers(custom_elements_);
   DOMWindow::TraceWrappers(visitor);
 }
 
 }  // namespace blink