Fix second button click moving slider while dragging

[Kruchji]

Fixes: ppy/osu#32765

[bdach]

I appear to still be able to break this:

Screen.Recording.2025-04-14.at.09.38.30.mov

1. Start dragging a slider bar that has a nub, but start the drag by clicking somewhere else than the nub
2. Drag some
3. Right click
4. Move mouse further
5. Nub will move away from where it should be

[bdach]

see above

[Kruchji]

Thanks for testing the behavior, I've hopefully fixed this case in the last commit. I've also included a second test for it - not sure if it's needed.

[bdach]

Thanks, seems to work well now 👍

That said I did apply a few small adjustments on my own in 0ddfd87 and 6462479; please check them when able.