Clear root window when the aura root window is moved/resized

ui/aura/root_window_host_linux.cc

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "ui/aura/root_window_host_linux.h"
 
 #include <X11/cursorfont.h>
 #include <X11/extensions/Xfixes.h>
 #include <X11/extensions/XInput2.h>
 #include <X11/extensions/Xrandr.h>
@@ skipping 493 matching lines @@
             p.SetPoint(0, 0);
           XWarpPointer(
               xdisplay_, None, x_root_window_, 0, 0, 0, 0, p.x(), p.y());
         }
         ConfineCursorToRootWindow();
       }
       if (size_changed)
         delegate_->OnHostResized(bounds.size());
       if (origin_changed)
         delegate_->OnHostMoved(bounds_.origin());
+      // TODO(oshima): Clear the root when the window is moved or
+      // resized while the extended desktop is disabled.
+      // crbug.com/152003.
+      if (base::chromeos::IsRunningOnChromeOS()) {
+        XSetWindowBackground(xdisplay_,
+                             x_root_window_,
+                             BlackPixel(xdisplay_, DefaultScreen(xdisplay_)));
+        XClearWindow(xdisplay_, x_root_window_);
+        // Set the background back to None to preserve content.
+        XSetWindowBackgroundPixmap(xdisplay_, x_root_window_, None);

[Daniel Erat, 2012/09/25 19:10:57]

it's not obvious to me why this is necessary.  can we just set the root's
background to black at startup when we're running on chrome os and be done with
it?

[oshima, 2012/09/25 19:33:28]

According to @marcheu, we intentionally set background pixmap to None, to
to avoid flashing when chrome switching to guest mode, or restarting
chrome due to crash.

[Daniel Erat, 2012/09/25 19:54:41]

Got it, thanks.  Wonder if it'd be cleaner to just use XFillRectangle() here. 
The downside is that you might need to create a GC first.  Yay X.

[oshima, 2012/09/25 20:45:09]

Done.

+      }
       break;
     }
     case GenericEvent:
       DispatchXI2Event(event);
       break;
     case MapNotify: {
       // If there's no window manager running, we need to assign the X input
       // focus to our host window.
       if (!IsWindowManagerPresent() && focus_when_shown_)
         XSetInputFocus(xdisplay_, xwindow_, RevertToNone, CurrentTime);
@@ skipping 495 matching lines @@
       ui::ViewProp::GetValue(accelerated_widget, kRootWindowHostLinuxKey));
 }
 
 // static
 gfx::Size RootWindowHost::GetNativeScreenSize() {
   ::Display* xdisplay = base::MessagePumpAuraX11::GetDefaultXDisplay();
   return gfx::Size(DisplayWidth(xdisplay, 0), DisplayHeight(xdisplay, 0));
 }
 
 }  // namespace aura