Canvas Scaling in Standalone Fullscreen (5.3)

My UI appears correctly in the editor, regardless of the screen size - it scales and lays out perfectly. When I build the app for Mac, it works fine in any resolution windowed. It also works when I use fullscreen at the same resolution I use for my desktop. But if I use a different resolution (if my desktop is set to 1920x1080, and I choose 1280x720, say), the canvas appears as if it were laid out for the desktop resolution: so it doesn't scale, and I just get the bottom left of the UI displayed. If I set my desktop resolution first, then run the app, it works.

I've tried all three Canvas Scaling modes, and all three Canvas Render Modes, no joy.

What else could I look for?

The strange inconsistencies suggest a bug to me, but I don't want to cry bug until I figure out whether I've missed something.