In Fullscreen mode (wayland, gnome 3.24.2, HiDPI), there is a distracting 1px margin at the bottom of the screen through which the content of the window under it can be seen.

I cannot reproduce, are you using Adwaita as a theme?
I usually use arc theme, but I've now tried with Adwaita and the issue is still present.
Do any other applications have the same issue, for example gedit?
No, I'm only having this issue with Tilix
Sorry, maybe it's a semantic issue, I meant "with the window maximised" not full screen (F11). The issue is not present in full screen.
Thanks, however I still cannot reproduce this. Is anyone else having this issue?
Wait! Sorry, I am having the issue with other apps like gedit! it's just that it was more obvious with tilix because it's got a dark background. I guess it must be a gtk/gnome bug then.