Server Version#: 1.14.0.5470
Player Version#: 2.23.0
In Grid View on higher resolutions (higher than 1200p) the slider breaks; it no longer resizes your posters.
It does not seem to be connected to the actual resolution of the screen, but how large the Plex Player window is. On 1440p with the window maxed, the slider does not work, but when making the window a smaller size, the slider starts working again. Quite odd, but possibly tied to some CSS math 
Did not happen on 2.22.0, probably tied to “Updated look of playlist/collection posters”.
EDIT: Same issue on a “native” web version (using Chrome).