I'm playing with KeeperFX 0.4.5 at fullscreen and at a INGAME_RES of 1280x960x32.
You can see a strange "layer" under the slabs selected to dig (these that are intermitent in the minimap). To see it well, move the mouse to the laterals to make the minimap move, you will see many white pixels move under the intermitent selected slabs.