I'm just getting started with Affinity Photo and have a long track record with using Photoshop for a lot of very low-level stupid things. Today's task is scanning music for use in a Keynote, and I need to get black text and notation on a transparent background. I use the non-contiguous flood selector to select all the white and near-white of the scan. Then I hit delete and... the white background is deleted. But it doesn't display the grey-and-white grid; the screen looks nearly identical to before. I can see in the layer thumbnail that all those white and off-white pixels have been deleted. When I save the file, it works just fine with a transparent background. But I can't see what's transparent and what's not while in Affinity Photo.
To add to my confusion and frustration, I somehow did something different with a different file, because in that one, the transparency grid is visible.
I'm sure I've unclicked a view option or something similarly simple, but I can't figure out what I've done differently.