All my mods are in a single skin_custom.less file, so I'll take that out and retry...
I'll report back. :)

I'm unable to reproduce on Metro M or Material Design, so it may be specific to your modded Skin.

When selecting albums in the Grid View, the top-right triangle and "isSelected" tick shape has a z-index that makes it display it above the columnHeaderBar (when scrolling), not below it...

[url=https://postimages.org/][img]https://i.postimg.cc/mkX6TpDX/image.png[/img][/url]

However, hovering over the album will correctly change the z-index to put it under the bar, but reverts to above when the mouse leaves the album.

Not sure if this is unique to my skin (a modded MetroM), or all skins exhibit this behaviour?
I haven't changed any z-index values with my CSS mods.