Group boundary not drawn properly in fastPaint or paint

Open an alignment with a group defined in the middle of the alignment (i.e. residues are visible above, below, left and right of the group). A project file is attached.

Two "modes" of odd behaviour are observed.
1) boundary artefacts.
Scroll down and up (slowly) with the group visible.
i) Horizontal lines are drawn between rows in the group where they shouldn't be (fastPaint -- these disappear when a refresh occurs [e.g. wiggle the mouse arrow over the alignment]);
ii) Left boundary lines only get drawn in fastPaint or refresh when the bottom of the group becomes visible;
iii) Right boundary lines do not get drawn in fastPaint or refresh.

2) sidelines in wrong place
Open an alignment, create an internal group, as described above.
i) scroll so that the top of the group is in the middle of the viewable area, but the bottom of the group is off the bottom. The side lines extend upwards from the top of the group (instead of down). Scroll further down, and when the bottom of the group appears, the side lines snap back into the correct place. Scroll up so the bottom disappears and the side boundary lines disappear again, revealing themselves to be again extending upwards from the top boundary line.