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

ui/views/widget/desktop_aura/desktop_window_tree_host_x11.cc

Index: ui/views/widget/desktop_aura/desktop_window_tree_host_x11.cc
diff --git a/ui/views/widget/desktop_aura/desktop_window_tree_host_x11.cc b/ui/views/widget/desktop_aura/desktop_window_tree_host_x11.cc
index 3c992813568aa97c66c5617ef3a75b1be63f99dd..781000ed1fdbd3f8784aa136cde909919522b269 100644
--- a/ui/views/widget/desktop_aura/desktop_window_tree_host_x11.cc
+++ b/ui/views/widget/desktop_aura/desktop_window_tree_host_x11.cc
@@ -958,8 +958,17 @@ void DesktopWindowTreeHostX11::SetCapture() {
 
   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() {