I've had this happen a couple of times today. Drove me nuts because I thought maybe my wireless keyboard was going sour, but a /reloadui fixed it. Though I've found that as soon as I have to start using /reloadui, the problems come back more frequently. So I eventually relog and it goes away for a few hours.
There are two very similar bugs of this kind and they belong to the "hard to track down and fix" kind.
One happens with certain combat abilities and can be fixed by /reloadui. Sometimes even jsut by entering/leaving mousemode and enter/leaving the chat message edit box.
The other seems to happen when trying to mount a horse. Happens most often in cities if I try to mount to early after using a door.
This one wil fix itself after time and maybe use of /stuck.
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