The bug
The one-pixel boundary of each slot in an inventory counted as part of the slot’s hitbox in previous versions, and no longer does in 26.4-snapshot-7. This creates a two-pixel wide lattice of dead space in all inventories, where clicking has no effect.
How to reproduce
Open your inventory
Click an item to pick it up
Hover your mouse between slots:
Click again
Expected result
The item should be placed in one of the slots
Observed result
Nothing happens
Notes
Separately, the hitbox for the hotbar slots seems to be shifted up by one pixel. As a result, the top pixel of its boundary is still selectable, but the bottom row of the slot as well as the bottom border pixel are not. I’ll report this separately if a fix for this issue does not address this (or if this is closed as WAI).
Confirmed. This also introduces some sort of flickering while cycling through items on the inventory, which wasn’t the case in previous versions.