I looked into this a bit more. It turns out I had already set the option in the project settings to “No change in camera size”.
The problem is that the black border GDevelop draw doesn’t actually change when the project’s resolution is changed! So there’s a preview screen that’s 800 by 600 pixels or so, and if you change your screen size to 850 by 600, it’s off by 50 pixels.
There is a workaround - enable the Mask feature under Grid. That shows the actual screen size. I have no idea why the default rectangle doesn’t.
Thanks for the help though, I wouldn’t have figured this out without having a second look at project properties!