Use XGrabPointer instead of XChangeActivatePointerGrab() to change the cursor during a pointer grab

ui/base/x/x11_util.cc

Index: ui/base/x/x11_util.cc
diff --git a/ui/base/x/x11_util.cc b/ui/base/x/x11_util.cc
index 03625c6de02f0782c1b8d3861f447c156daa0377..72e0e80912cbd619191e1076b88a95fa9c7dd501 100644
--- a/ui/base/x/x11_util.cc
+++ b/ui/base/x/x11_util.cc
@@ -284,6 +284,16 @@ bool IsXInput2Available() {
   return DeviceDataManagerX11::GetInstance()->IsXInput2Available();
 }
 
+int GrabPointer(XID window, bool owner_events, ::Cursor cursor) {

[pkotwicz, 2014/12/22 23:44:16]

The semantics of DesktopWindowTreeHostX11::SetCapture() do not allow something
like ScopedGrabPointer.

By design, UngrabPointer() is not called if capture is transferred from one
WindowTreeHost to another.

I have added comments in DesktopWindowTreeHostX11::SetCapture() and in
x11_util.h

[sadrul, 2014/12/23 16:53:50]

How does this transfer happen? Can it be done by transferring ownership of the
scoped object?

[pkotwicz, 2014/12/23 18:22:22]

The transfer occurs internally in X. The transfer is initiated via calling
XGrabPointer() with a different X window. Because everything in X is
asynchronous, the transfer of capture is also asynchronous. This transfer is
documented in http://tronche.com/gui/x/xlib/input/XGrabPointer.html
"XGrabPointer() overrides any active pointer grab by this client."

No, the transfer cannot occur by transferring ownership of a scoped object.

Aside: Calling XUngrabPointer() if the pointer does not have mouse grab is
perfectly safe.

Where do you suggest putting these methods? I am not adverse to having these
methods in x11_util.cc 99% of the time it is wrong for Chrome code to call
directly into X11 so we should avoid including x11_util.h if at all possible.

[sadrul, 2015/01/14 20:34:01]

Where in the chrome code do we attempt to transfer the capture from one WTH to
another?
Somewhere in ui/views/widget/desktop_aura/ (e.g. perhaps x11_pointer_grab.cc|h)

[pkotwicz, 2015/01/14 20:50:07]

We transfer capture from one WTH to another when dragging tabs

+  int event_mask = PointerMotionMask | ButtonReleaseMask | ButtonPressMask;
+  return XGrabPointer(gfx::GetXDisplay(), window, owner_events, event_mask,
+                      GrabModeAsync, GrabModeAsync, None, cursor, CurrentTime);
+}
+
+void UngrabPointer() {
+  XUngrabPointer(gfx::GetXDisplay(), CurrentTime);
+}
+
 static SharedMemorySupport DoQuerySharedMemorySupport(XDisplay* dpy) {
   int dummy;
   Bool pixmaps_supported;