The message is still contained. The image used for .timedMessage is only 200px x 200px so it can't cover the area. That is a quite long Board Closed message, wouldn't that sort of message be better as a announcement or similar?
I don't think it's really worth changing anything in particular here. It's an edge case requiring a long board closed message and doing something that triggers that sort of response when closing the board. Arguably, this could be considered an error case but we've always returned the board closed as just a "message" so I don't see that changing in the short term.