X11: How do I REALLY grab the mouse pointer?

Question

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);

Answer 1

I'm almost embarrassed to admit this, but the answer turned out to be as simple as changing that third parameter (owner_events) from True to False.

From the Xlib tutorial/reference at tronche.com:

If owner_events is False, all generated pointer events are reported with respect to grab_window and are reported only if selected by event_mask. If owner_events is True and if a generated pointer event would normally be reported to this client, it is reported as usual. Otherwise, the event is reported with respect to the grab_window and is reported only if selected by event_mask.

I'm still not sure I completely understand the behavior of the True case, but my interpretation is that if True, other X programs (that is, windows created by other processes) are barred from receiving XEvents, but those that would hit any window created by your process are delivered as normal. In the false case, all events are reported with respect to the specific window you have selected. This was the behavior I was after.