[6.x] More elegantly mask horizontal overflow for sets

[jaygeorge]

Description of the Problem

This is really a tiny indulgent "UI polish" PR, rather than a "problem to be solved".
#14746 added an overflow mask for longer set header names, which works well.
However, the scroll bar was also "masked" at the end, which is a bit like pulling back the curtain to see the Wizard of Oz pulling levers.

What this PR Does

• Enhances the overflow with a tiny scroll-driven animation to remove the mask when we reach the end, so that the scrollbar does not get caught up in the mask
• Turns this into an editable utility in case we want to use it elsewhere

Before

The scrollbar is also clipped by the mask

After

The mask is still in place, but doesn't affect the scrollbar

How to Reproduce

1. Add a set with a long name
2. Open Live Preview, or view the replicator handle in a small space