It seems this bug only happens after a tab is detached, after a few minutes, or some random period, I still don't know the exact trigger. How I detach a tab: In irc, right click on a tab where the channel name is, then select "detach this tab."
I'm also using xmonad for a window manager, since this relates to a window, it could be relevant. I'm not going to test it under another window manager, it's too disruptive for me. I'm just not going to detach tabs anymore, so this bug's severity can be lowered.
