Fix grabbing capture when the mouse is pressed on Desktop Linux.

ui/views/widget/desktop_aura/desktop_window_tree_host_x11.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/views/widget/desktop_aura/desktop_window_tree_host_x11.h"
 
 #include <X11/extensions/shape.h>
 #include <X11/extensions/XInput2.h>
 #include <X11/Xatom.h>
 #include <X11/Xregion.h>
@@ skipping 940 matching lines @@
   // - |g_current_capture|'s X window has capture.
   // OR
   // - The topmost window underneath the mouse is managed by Chrome.
   DesktopWindowTreeHostX11* old_capturer = g_current_capture;
   g_current_capture = this;
   if (old_capturer)
     old_capturer->OnHostLostWindowCapture();
 
   unsigned int event_mask = PointerMotionMask | ButtonReleaseMask |
                             ButtonPressMask;
-  XGrabPointer(xdisplay_, xwindow_, True, event_mask, GrabModeAsync,
-               GrabModeAsync, None, None, CurrentTime);
+  int success =
+      XGrabPointer(xdisplay_, xwindow_, True, event_mask, GrabModeAsync,
+                   GrabModeAsync, None, None, CurrentTime);
+  if (success == AlreadyGrabbed) {

[sadrul, 2014/11/24 15:46:01]

Can you link to some doc here? man XGrabPointer claims AlreadyGrabbed is
returned when some other client has a grab, and it doesn't sound like that's
what you are saying in the comment below.

[pkotwicz, 2014/11/24 16:07:41]

The docs are not great. However man XUngrabPointer mentions: " The
XUngrabPointer function releases the pointer and any queued events
 if this client has actively grabbed the pointer from XGrabPointer, XGrabButton,
or from a normal button press."

I suspect that "if the pointer is grabbed from a normal button press", this
counts as a different client. Otherwise, XGrabPointer should not fail according
to the spec. In particular the spec mentions: "XGrabPointer overrides any active
pointer grab by this client."

[sadrul, 2014/11/27 17:48:18]

Do we have example of any other clients doing similar things? (from a quick
glance, GTK+ doesn't seem to do this)

+    // Try ungrabbing the pointer and regrabbing it. If the OS grabbed capture
+    // as a result of a mouse button being pressed, ungrabbing and regrabbing
+    // the pointer will succeed.
+    XUngrabPointer(xdisplay_, CurrentTime);
+    XGrabPointer(xdisplay_, xwindow_, True, event_mask, GrabModeAsync,
+                 GrabModeAsync, None, None, CurrentTime);
+  }
 }
 
 void DesktopWindowTreeHostX11::ReleaseCapture() {
   if (g_current_capture == this) {
     // Release mouse grab asynchronously. A window managed by Chrome is likely
     // the topmost window underneath the mouse so the capture release being
     // asynchronous is likely inconsequential.
     g_current_capture = NULL;
     XUngrabPointer(xdisplay_, CurrentTime);
 
@@ skipping 954 matching lines @@
   if (linux_ui) {
     ui::NativeTheme* native_theme = linux_ui->GetNativeTheme(window);
     if (native_theme)
       return native_theme;
   }
 
   return ui::NativeTheme::instance();
 }
 
 }  // namespace views