On high-DPI displays, VNC connections seem to have a hard-coded viewport height
I have a laptop with a 2880 x 1800 display, which I use with 2x scaling. This gives me a 1440 x 900 usable area with everything.
When I use GNOME Connections to connect to a remote machine through VNC, the viewport refuses to stretch beyond a certain height. There doesn't seem to be a limit to how wide it will go, but beyond a certain point the viewport just stops and there's a grey bar across the bottom of the GNOME Connections window.
I've attached two screenshots of my laptop connection to a 4.5K iMac, with display scaling switched on and off. These were taken on GNOME Connections 42.1.2.
Will test in nightly and report back in a few minutes...