To summarize, this is extremely simple:
- the mouse scroll actions should be added to the meter spanning over the visible area of the lyrics (here, that would be [LyricsBACKGROUND])
- the principle is incrementally changing the vertical coordinate where the lyrics text is displayed (here, #LyricsY# which is basically [lyrics:Y])
In other words, we "move" the lyrics text up or down as needed, while the [LyricsBGContainer] container meter hides what's outside the visible area.
The Y of the [lyrics] meter is initially 0 because the position of [lyrics] is relative to that of the container meter. It goes negative when scrolling down because then we place [lyrics] at a vertical coordinate "upside" of the container area (in Rainmeter, vertical coordinates increase downwards).