Bug 341148

Multi-pointer X (MPX) was an addition to X11 that allowed there to be multiple on-screen mouse pointers, with virtual and physical mice and keyboards attached to them (comparable to the functionality of multi-touch touchscreens)


Reproducible: Always

Steps to Reproduce:
1. (optional) Press Shift+Alt+F12 to disable compositing.
2. (optional) run 'xinput list' between other xinput commands to see what changed.
3. (optional) Plug in a second mouse (serial if you know your way around inputattach, or PS/2 or USB)
4. (optional) Also another keyboard, if you have the second mouse (having two keyboards with one mouse makes it hard to focus one of the pointers - if the keyboards aren't attached to the same one)
5. xinput create-master "Virtual core 2"
6. (optional) xinput reattach "pointer:Logitech USB Receiver" "Virtual core 2 pointer" (use the actual name of your mouse or the id - you only need 'pointer:' if there's a keyboard with the same name, and you need to use the id if there are two mice with the same name)
7. (optional) xinput reattach 9 "Virtual core 2 keyboard" (again, use your exact id or name)
8. Re-enable compositing.
9. (optional) System Settings→Desktop Effects→Show Paint - this helps with measuring how much it flickers

Actual Results:  
The original mouse pointer is still drawn the same, solid, over frame redraws, windows, and the desktop.
The second mouse pointer is drawn differently than the original:
• The whole screen may be painted in the same color, with the secound pointer not visible.
• The whole screen may be painted in the same color, with the second pointer visible on top of it.
• The whole screen may be painted in the same color, with a square of a different color of paint around the second pointer, with the second pointer visible.
If there is something redrawing the screen (a cpu monitor or systray chat client trying to get your attention), the mouse flickers while it is changing between these various states of drawnness.

Expected Results:  
Both pointers being drawn solidly, on top of everything else, responding to whatever physical mice and keyboards are attached.

Steps to return to normal:
Disable System Settings→Desktop Effects→Show Paint
xinput remove-master "Virtual core 2 pointer" AttachToMaster

I remember, sometime in the past, if I created the second mouse pointer while the compositor was running, I could disable the compositor, stopping the flicker, and enable it again, which wouldn't make it flicker again. - If the compositing was turned on with two mice, it would work fine. If the second mouse was added while it was running, it had to be restarted to notice it. Now this restart doesn't help. This was before the AttachToMaster option existed.

Workaround #1: Don't use the compositor.
Workaround #2: Never use more than a single mouse pointer.
Workaround #3: Don't use the second mouse pointer for anything that requires knowing its position in a short amount of time.

My guess is that it's some sort of rendering synchronization problem. Some frames have both mice, some have one.