(latest version of Affinity Designer 1.7.1 from the Apple Store, OS X 10.10 with the "latest" updates)
After changing the document dimensions with "File > Document Setup…" I experience refresh issues on the background area of the document (doesn't seem to matter if the background is transparent or a color). This especially happens very often after adding extra space to a document by using the "Anchor to Page" option. Perhaps related to the fact if it has to trigger a new zoom level afterwards or not. Usually on the right side of the document parts are missing then or I get bars in the UI background color. See the vertical bar in the screenshot (no such rectangle part of my test document).
Moving the document around in the viewport will repaint those areas properly.
This is easily reproducible.