mus: Fix ash::GetRootWindowController for child windows/widgets

ash/root_window_controller.cc

Index: ash/root_window_controller.cc
diff --git a/ash/root_window_controller.cc b/ash/root_window_controller.cc
index d4e9d54f46839f0cdd8618e43dbd7a68b361a19d..fe3e3e98a699c53db23fe56ce12c5a62ea7995b5 100644
--- a/ash/root_window_controller.cc
+++ b/ash/root_window_controller.cc
@@ -1056,7 +1056,21 @@ void RootWindowController::OnTouchHudProjectionToggled(bool enabled) {
 
 RootWindowController* GetRootWindowController(
     const aura::Window* root_window) {
-  return root_window ? GetRootWindowSettings(root_window)->controller : NULL;
+  if (!root_window)
+    return nullptr;
+
+  if (Shell::GetInstance()->in_mus()) {
+    // Look up the root for the display nearest |root_window|.
+    display::Display display = display::Screen::GetScreen()->GetDisplayMatching(
+        root_window->GetBoundsInScreen());
+    aura::Window* root_window_for_display =
+        Shell::GetInstance()
+            ->window_tree_host_manager()
+            ->GetRootWindowForDisplayId(display.id());
+    DCHECK(root_window_for_display);
+    return GetRootWindowSettings(root_window_for_display)->controller;
+  }
+  return GetRootWindowSettings(root_window)->controller;

[oshima, 2016/05/12 00:40:56]

This behaves differently if the most of the root window is located outside of
the root window. I *think* this will not happen in *ash UI, although I'm not
100% sure.

I wonder if we should at least use the transient parent's root window?

[James Cook, 2016/05/12 16:43:12]

I'm sorry, I don't completely understand.

I understand in mus ash that if a widget's window is mostly outside the root
window for display A it might choose display B's root window. However, if
|root_window| really is the root of the display then it should return the
correct RootWindowController, right? Or can the display's root window be mostly
outside the display?  Should I use the origin of the window?

I don't understand what you're suggesting about transient parents. Since this is
a "root window" on mus it should be the top-level aura window for a widget. I'm
not sure how the transient parent helps.

The behavior here in classic ash is identical to the old code.

 }
 
 }  // namespace ash