i3 does not fill screen after removing external monitor

I'm experiencing a problem with i3 4.17.1 2019-08-30 (on Ubuntu 18.04) on a Thinkpad T480s with Intel graphics.

I often use the laptop with external monitors (HDMI/DP via a dock). Sometimes when I dock/undock, and run xrandr/autorandr to re-set the monitor settings, i3 doesn't to get confused, and I end up with strange situations where the X display seems offset from the i3 display. No amount of xrandr, switching to TTY, suspending/resuming, or restarting i3 seems to get me out of the weird state.

It's hard to explain, so here's a picture: https://imgur.com/a/jbg2uB6 showing the X desktop filling my laptop screen, but i3 only filling part of it (the status bar and the bottom of the window are off the bottom of the screen). The Chrome window is fullscreen. The mouse pointer can move around the screen (including the grey area) but clicks register offset from where the pointer is.

This doesn't make any sense to me. xrandr is correctly reporting 1920x1080+0+0, there is no panning or larger virtual screen stuff going on. All my drivers, X, i3 are up to date.

I'm posting this here rather than registering it as an issue on github because I haven't yet established that the problem is i3... but I have tested with Gnome, KDE, OpenBox and the problem does not occur there.

I love i3 and have used it for years, but if I can't fix this I'm going to have to find a more reliable option. Does anybody have any ideas please?

Edit: I should say, I think the X/xrandr side of things is OK - the resolution matches my panel and my mouse hits the edges of the laptop display (there's no X desktop off to the sides etc). It's just that i3 doesn't seem to recognise the correct X dimensions.