Fix for out-of-sync scale factor issue

chrome/browser/ui/startup/startup_browser_creator_impl.cc

Index: chrome/browser/ui/startup/startup_browser_creator_impl.cc
diff --git a/chrome/browser/ui/startup/startup_browser_creator_impl.cc b/chrome/browser/ui/startup/startup_browser_creator_impl.cc
index 451b0e5c5b8b01fc7719ceb7a846a9a56cd9f499..d9413dce7c7e8a20521d1c0cee59a1ce10b06168 100644
--- a/chrome/browser/ui/startup/startup_browser_creator_impl.cc
+++ b/chrome/browser/ui/startup/startup_browser_creator_impl.cc
@@ -95,6 +95,8 @@
 
 #if defined(USE_ASH)
 #include "ash/launcher/launcher_types.h"
+#include "ash/shell.h"
+#include "ui/aura/window.h"
 #endif
 
 #if defined(OS_MACOSX)
@@ -806,6 +808,14 @@ Browser* StartupBrowserCreatorImpl::OpenTabsInBrowser(Browser* browser,
 #endif
   }
 
+#if defined(USE_ASH)
+  // Set the browser's root window to be an active root window now
+  // so that that web contents can determine correct scale factor for
+  // the renderer. crbug.com/155201.
+  ash::Shell::GetInstance()->set_active_root_window(

[sky, 2012/10/12 14:31:40]

I don't understand why this is necessary. Why is the renderer keying off the
active RootWindow? Wouldn't this mean if a timer kicks in and tries to create a
renderer that isn't in the active rootwindow it'll get the wrong scale?

[oshima, 2012/10/12 16:04:46]

We're not passing enough information for WebContentsViewAura::CreateView
to be able to tell in which display/scale factor it is being created. Thus,
window_->SetParent(NULL) in this method will pick active root window.

And when you create a new browser on 1x display, while an active root window
is on 2x display, it will first create renderer with 2x scale factor, and then
switch to 1x when it is added to a browser. And there is a potential race
condition when you switch scale factor right after its created, which can result
in showing 1x on the web contents. (refreshing by moving/resizing fixes
the issue, according to the report). Even without the race, it'd be waste to
render at different scale factor and then drop the result.
It will use the correct scale but issue is the race and waste, but above race
happen only when you switch immediately after it's created. Waste may still
happen though.

RIght fix is to pass enough information to WebContentsViewAura::CreateView
so that it can determine the correct scale factor, but it will requite a lot of
changes in many places, and it's risky to merge to 23. I filed a bug
(crbug.com/155443) and will work on this for 25.

And I'll update the comment to be more clear.

+      browser->window()->GetNativeWindow()->GetRootWindow());
+#endif
+
   // In kiosk mode, we want to always be fullscreen, so switch to that now.
   if (CommandLine::ForCurrentProcess()->HasSwitch(switches::kKioskMode))
     chrome::ToggleFullscreenMode(browser);