OK, this is a wierd one:

I was in the crumbling dungeon with a tension room somewhere. I zapped a wand of monster detection to see if the door in front of me was safe--it was. The mini-map showed the tension room in the opposite corner of the dungeon. So I recharged the wand, then:
1. switched to ascii mode
2. zapped the wand, looked at the monsters
3. switched back to tile mode

At this moment, the message buffer showed the messages from dipping and zapping the wand, but without line breaks in between. Then I started exploring, and the game kept adding new messages to the buffer without line breaks. The whole buffer looks like one long sentence, so it just kept expanding higher and higher on the screen (both up from the bottom left and down from the top).

Saving the game and restoring reset the buffer, but doesn't solve the problem.

Save game and screenshot attached.

EDIT: Restoring the game after saving, without exiting ADOM, did not fix the problem. However, completely exiting the application, restarting, and restoring the game did resolve it.