I've implemented a horizontal splitter widget in Xlib. I'm trying to grab the mouse when the user clicks & drags on the splitter bar (so that the user can dynamically move the split & thus resize the windows on either side of the splitter bar).

I've used XGrabPointer() after receiving a left click, in hopes that all future mouse motion (dragging) will be diverted to the splitter window until the left button is released.

Unfortuntately, it doesn't seem to work like that. If the user drags too quickly and the mouse pointer enters one of the windows on either side of the split, the MotionEvent messages are diverted to that (child) window rather than the splitter window.

What have I done wrong? My XGrabPointer() call is as follows:

::XGrabPointer(mDisplay, window, True,
               ButtonPressMask |
                 ButtonReleaseMask |
                 PointerMotionMask |
                 FocusChangeMask |
                 EnterWindowMask |
                  LeaveWindowMask,
               GrabModeAsync,
               GrabModeAsync,
               RootWindow(mDisplay, DefaultScreen(mDisplay)),
               None,
               CurrentTime);