I encountered this problem when another player had been playing from the same computer and had disconnected due to inactivity while in a shop. When I logged into the game afterward his shop interface was still on the screen but inactive and unclickable. Logging in and out, zoning, opening and closing my own shop, and exiting full screen all failed to remove it, a client restart was needed to do so.

Overall the glitch only amounts to a minor annoyance but I thought it warranted mentioning.