Where did this happen?
In the first combat after the 2nd "green box" zone transition (against several large Deep Spiders).
What did I do?
Several turns into combat I was controlling Garrad. I switched from Sword & Shield to Sword & Torch in an attempt to light the nearby oily rocks on fire and improve visibility. Hovering my mouse over the rocks I noticed that the mouse-over icon showed a sword rather than a gear. Not processing this in time I clicked on the oily rocks, Garad swung at them, and the game's bottom UI disappeared and did not return. I could still open the menu and reload an earlier save, however.
Is it reproducible?
I tried to reproduce the scenario but was unable to. I hope this bug report can still be of some use regardless.