Confirmed - cause found, patch not written yet.

The authentic 54x18 button sprite is being drawn into a 17px plate, so it overruns the bottom bar's top rule.

Moving it out of OPEN so the list reflects reality: this one is pinned down to exact files and lines, so what is left is writing and testing the change, not working out what is wrong.

Fixed in b16ca39e.

Cause: the footer band was mis-measured by one pixel and the sprite by one row, and the two compounded into the overrun you saw.

Reading the equipment window art row by row: the last slot row's boxes end at y264, the empty footer plate runs y265-281, and y282 is a solid rule - and that rule is not part of the plate, it is the line that OPENS the window's bottom bar. The code's comment claimed the plate was 18 rows; it is 17.

The button sprite is 54x18 in every state, but the button inside it is 53x17: the outline and green gradient occupy rows 1-16, row 0 and column 0 are an antialias edge, and the far column plus the bottom row are a 1px light DROP SHADOW. Drawn whole at y265, that shadow row landed on y282 and smeared 50px of the bar's opening rule - which is what you were seeing.

Fix: the button is drawn at its real size inside the real footer band, so it sits within the plate with the bar below it untouched - smaller, and in the bottom-left corner, as you asked.

Retest: open the equipment window and check the PET EQUIP button and the bar beneath it at 1x.