I see how this was pretty confusing. In the latest update all borders use Rhino.UI.ThemeSettings.Frame.Edge:
Meaning we get borders with 3, 2, and 1 pixels widths when collapsing the left panel:
IMO this looks weird/unintentional, especially when using a darker color like here. Which could be fixed by using Rhino.UI.ThemeSettings.Content.Background for the two 2 pixel wide interior borders (i.e. similar to the horisontal interior border in the previous release):
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