Weirdness (?) when moving vertical separator in demo application.

When I try to select and drag the vertical separator between the tree node pane on the left hand side and the rest of the application, black vertical bars seem to be drawn at an offset to the vertical separator being dragged.

Note that this is observed with WxPython4 toolkit backend.

(I couldn't find a better way to describe the issue. If anyone else knows of better/right words, please feel free to update the issue or comment below)