This address #2102:

The test case there (Thank you @pijyoi!) triggers a case I had not considered in the range adjustment code of ColorBarItem:

• Set the (top) range and limit to a value that is slightly below a rounding step.
• Start adjusting the bottom range

Result:
The top end of the range is detected out of range and corrected down, but restored to the original value by rounding.
This is (continuously) interpreted as both top and bottom end moving, a "region move" normally triggered by dragging the center of the bar.
In that case, the previous size of the region is held fixed to avoid squishing the range against the top or bottom limit.
Which is the bug encountered in #2102 : The region actively resists any attempt to make it smaller.

This PR

• slightly expands the limits to make them consistent with the rounding step
• double checks for a region move before blocking the region size.

I hope that eliminates the issue in the future and closes #2102