Non-modal UI.
I accidentally both left-clicked and right-clicked an item in my inventory at (almost?) the same time, which caused the part description window to appear, and i was also holding the item in my mouse cursor, which could then used like normal, without removing the part description window. (Dropping it on the ground, swapping it with equipped parts...)
The bug is that i could move an inventory part around with the mouse while its description window was active, which is normally impossible. This also means it's possible to take time-based actions with a part window active, which i don't think is also normally possible?
(I wonder if there are more bugs that can result from this, like by dropping an item that then gets damaged/destroyed by for instance explosives, all while its description window is still open?)