Just worked in "lights out" mode (development module) and wanted to export a photo. I noticed that the export dialog is partially obscured like the main LR window - screenshot:
Happens in 3.4.1 and 3.5RC. Does _not_ happen in full-screen mode. Also happens for the settings dialogs and the import (!) dialog. After switching to other applications and back, the dialogs become completely obscured (really confusing if you are in 100% lights out - screen goes completely black). Also, moving the dialogs causes some different areas of the dialogs to be affected by the lights out.
Win XP 32 Bit SP3, 1600x1200 screen resolution. Edit: Video: NVidia GeForce 7600 GS, latest driver from NVidia website.