Thanks, Zinglish. You were right. I also discovered the issue with the gutter width: it has it’s own value at the very bottom of the Style section for width. Though, I discovered that hiding it altogether and positioning a progress bar, bound to the same variable, directly behind the slider, made for a much better slider:
The one thing I haven’t solved, and don’t think there is a solution for, is that with such a wide thumb button, it doesn’t behave exactly as intended, regarding it’s hit region: when the slider is at 0%, then the bottom of the thumb is where a narrow thumb would be, meaning that clicking on the center of the thumb makes it jump up to 10% or so. But when it’s at 100%, then the top of the thumb is at 100%, so clicking on the center jumps it down to 90% or so.
It’s not a huge issue. The obvious work around is to do something similar to what I did replacing the gutter with a progress bar: use the default thumb, and bind the big thumb as an image whose screen offset is bound to the same variable, stack it on top of the slider and prevent it from catching any hits. Not sure how all that affects performance, though.