I'm a long time user of Pixelmator and one of the things I really like about Pixelmator Pro is the ablilty to scroll beyond the edge of the canvas. It has a lot of uses. However at some levels of zoom, usually when the canvas is just filling the space available for it, the borders disappear and I can't pan beyond the canvas edge. Hopefully you can recreate this easily. If not, let me know and I'll come up with a scenario that should recreate it.
All the best,
- Stef.
(Using Pixelmator Pro 1.0.8 on macOS 10.13.3 on a MacBook (Retina, 12-inch, Early 2015) )