Problem summary
The Swipe tools produce black artefacts in the Monitor when an image is used as Overlay or Underlay.
Steps to reproduce
1) Open the Hello_pony example scene (Help > Examples > Hello Pony PRMan).
2) Do an interactive render at the Gaffer level.
3) Click on the toggle overlay/underlay controls (top right corner options in the Monitor).
5) Middle mouse drag the image from the Front thumbnail of the Monitor to the Underlay.
6) Activate the Swipe line.
7) While hovering in the Monitor, middle mouse drag the image around (panning) and you can see there is a black shape created that oscillates in size as you move around.
8) Select the swipe line tool and move it around the image, same shapes are shown and change in size as you move across the image in the Monitor.
If you don't use anything as underlay/overlay, the swipe tool works as expected.
Workaround
Unknown.
Reproduced by support
Reproduced in Katana 1.0v6,1.2v2.
2.0v1, 2.0v5, 2.1v1, 2.1v4 (Linux), 2.5v1, 2.5v2 (Linux and Windows 7)
Expected behaviour
When used with an Underlay or Overlay, the Swipe tools should still produce accurate results in the Monitor.
Actual behaviour
The Swipe tools introduce artefacts.