I made a quick recording that demonstrates the offset thing: as I drag the mouse, note the position of the pointer relative to the slider: it “stretches”:
(the skip in the middle was just lag).
<edit> this post will make sense once my previous response, which is awaiting moderation, is released. Short version is that I found the problem with the gutter width, but have a separate issue with the large thumb and the hitbox location. </edit>