Xlib XEventQueue doesn't always contain all events

Question

hopefully, this is not a bug and just my misunderstanding of how Xinput is supposed to work.

I have been writing a game, and I am using Xlib to get input from the keyboard. The bizarre thing is that some KeyRelease events seem to get stuck in the XServer indefinitely until another event happens and pushes the event to the queue.

I can get this to occur if I mash 3 buttons at once, (Left, Up, and Right arrows) repeatedly. Ironically I have only ever been able to get it to happen with the Left arrow key. But I have reproduced it on different machines.

I have used XPending() to check the event queue, as well as XCheckMaskEvent() and both of them, seem to think the queue is empty. But sure enough, when I press another key such as right the KeyRelease event for left happens at the exact same time.

Is this a bug? Or expected functionality?

• Side note, it might be useful to know that I am grabbing the keyboard on the root window

Here is the actual code

loop do
  Fiber.yield
  next if (event = display.check_mask_event(KeyPressMask | KeyReleaseMask)).nil?
  case event
  when KeyEvent
    kcode = event.lookup_keysym event.state & ShiftMask ? 1 : 0
    return {event.type, kcode}
  else
    next
  end
end