@s-kyy, @jellishero and anyone else with this problem: The only way I could replicate it myself was by shrinking the window so that the left sidebar was too tall for the window. This is clearly not what was happening in @s-kyy’s screen recording, but it did cause the same whole-ui-scrolling effect, and I was able to come up with a fix:
html, body { overflow: hidden; }
You can see it working in the following screen recording: