Better Lyrics thumbnail overlay compatibility

[Sv443]

Link to the extension or userscript:

https://addons.mozilla.org/en-US/firefox/addon/better-lyrics/

Details about the incompatibility:

When the thumbnail overlay is active, it obscures the lyrics in full screen.

[Sv443]

To fix the layering, ytmusic-player-page #side-panel needs a higher z-index.
To fix the thumbnail overlay being shifted, :root #bytm-thumbnail-overlay-bg-img needs fixed positioning with top and left. And since the blur effect makes the gradient background bleed over, the thumbnail could be expanded off-screen in each direction.