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

ui/base/x/x11_util.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.
 
 // This file defines utility functions for X11 (Linux only). This code has been
 // ported from XCB since we can't use XCB on Ubuntu while its 32-bit support
 // remains woefully incomplete.
 
 #include "ui/base/x/x11_util.h"
 
@@ skipping 266 matching lines @@
   std::map< ::Cursor, XCustomCursor*> cache_;
   DISALLOW_COPY_AND_ASSIGN(XCustomCursorCache);
 };
 
 }  // namespace
 
 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;
   // Query the server's support for XSHM.
   if (!XShmQueryVersion(dpy, &dummy, &dummy, &pixmaps_supported))
     return SHARED_MEMORY_NONE;
 
 #if defined(OS_FREEBSD)
   // On FreeBSD we can't access the shared memory after it was marked for
   // deletion, unless this behaviour is explicitly enabled by the user.
@@ skipping 1198 matching lines @@
       << "request_code " << static_cast<int>(error_event.request_code) << ", "
       << "minor_code " << static_cast<int>(error_event.minor_code)
       << " (" << request_str << ")";
 }
 
 // ----------------------------------------------------------------------------
 // End of x11_util_internal.h
 
 
 }  // namespace ui