Rhino 6.12 - MoveUVN flaw

When I drag any edge of MoveUVN panel, rectangle (shown in the following screenshot) appears on the left side of the panel.

Works here in
Version 6 SR13
(6.13.19031.5531, 1/31/2019)
and in
Work In Progress
(7.0.19031.1365, 1/31/2019)