A picture + freehand 'circle' says more than a thousand words:
You can reproduce it e.g. by clicking here and scrolling down. Expected behaviour is that the pieces disappear behind the bar, just like the board does. The problem is that both the pieces (
.chess-replayer .board .chess-piece) and the review bar (
.review-bar-container .review-bar) have a
z-index value of 10. Changing the
z-index of the pieces to 9 solves the problem and does not introduce other issues, as far as I can see.