xcb: LeaveNotify received immediately after EnterNotify

Question

I am in the process of writing a window manager in Rust for learning purposes, using the xcb library. My code and several test windows (xterm instances) are all running inside of a Xephyr session. I set my event mask on the root window as

xproto::EVENT_MASK_SUBSTRUCTURE_REDIRECT
| xproto::EVENT_MASK_SUBSTRUCTURE_NOTIFY
| xproto::EVENT_MASK_POINTER_MOTION
| xproto::EVENT_MASK_LEAVE_WINDOW
| xproto::EVENT_MASK_ENTER_WINDOW
| xproto::EVENT_MASK_BUTTON_PRESS
| xproto::EVENT_MASK_PROPERTY_CHANGE
| xproto::EVENT_MASK_FOCUS_CHANGE

and all child windows have an event mask of

xproto::EVENT_MASK_ENTER_WINDOW
| xproto::EVENT_MASK_LEAVE_WINDOW
| xproto::EVENT_MASK_BUTTON_PRESS
| xproto::EVENT_MASK_PROPERTY_CHANGE
| xproto::EVENT_MASK_POINTER_MOTION
| xproto::EVENT_MASK_FOCUS_CHANGE
| xproto::EVENT_MASK_STRUCTURE_NOTIFY
| xproto::EVENT_MASK_EXPOSURE

When I move the mouse over a non-root window, I use the EnterNotify event to grab mouse buttons on that window, for click-to-focus purposes, and ungrab on LeaveNotify. The sequence of events is:

• Mouse on root window
• Move mouse over non-root window
• Receive EnterNotify for non-root window
• Grab buttons on the non-root window
• Immediately receive LeaveNotify, while the mouse is still over the non-root window
• Ungrab buttons due to LeaveNotify
• Attempt clicking on the non-root window
• Receive a second LeaveNotify for the non-root window
• Receive an EnterNotify for the root window, and grab mouse buttons on the root window
• ButtonPress is sent for the root window, despite the cursor being over a non-root window

I'm genuinely unsure what the cause of this might be; using Google and the like has turned up nothing useful.

---

For anyone who might stumble across this later, a partial solution is:

• Only listen on window enter events to grab buttons on a window
• Mask root to be SUBSTRUCTURE_REDIRECT | SUBSTRUCTURE_NOTIFY | BUTTON_PRESS only
• Don't mask LEAVE_WINDOW onto non-root windows, and don't handle those events

I don't want to add this as an answer because:

1. I'm unsure it's the correct way of doing it.
2. I've not tested this thoroughly.
3. It doesn't answer my primary question of "why are events received like this?"

Answer 1

Take a look at the protocol reference manual. It describes (among lots of other things) the exact algorithm for generating enter and leave events: https://www.x.org/releases/X11R7.6/doc/xproto/x11protocol.html#events:pointer_window

In your specific case, I would expect the LeaveNotify event to have mode: Grab, which means that the window no longer has the "normal" pointer focus, because something (your program) grabbed the input.

If this is not the answer, I can recommend running your WM under xtrace / x11trace (available in Debian-based distros as package xtrace). This program prints all X11 traffic that "goes through". This might help figuring out what is going on.