https://i.imgur.com/tnfKPT6.png
Bottom row, 2nd from right.
In shop screens and stash, in Legacy 640x480 mode that particular inventory spot is hard to click (the very bottom of the square seems clickable, but not the whole square like the rest)
It seems to only affect 640x480 Legacy (800x600 Legacy is fine, as is both resolutions at Resurrected)