Workspaces are lost when connecting an external monitor

[Paride Legovini]

Steps to reproduce:

1. On a laptop, connect an external monitor and set it up as the "primary display" in a "Join Displays" setup.
2. Setup a few workspaces in the primary display by placing windows in them, e.g. a few terminals.
3. Disconnect the external monitor. The workspaces with their windows are transferred to the build-in monitor, as expected. The number of workspaces stays the same, each workspaces still has the windows it contained before, on the external monitor.
4. Reconnect the external monitor. The previous dual-monitor configuration is automatically applied.
5. The workspaces move from the built-in display to the external monitor (which is again the primary monitor), but in this process some (sometimes all) workspaces are lost, and the windows get mixed in the remaining workspaces. The user's setup is lost. I experience the issue almost every time.

Expected outcome: all the workspaces are retained when connecting a different primary monitor, each window stays in the same workspace it was before.

My feeling is that, when the external monitor is connected, there is a race condition between moving the workspaces and moving the windows to it. Sometimes a window is moved before its workspaces has been created, and ends up in a different workspace. The problem is very easily reproducible but not deterministic.

Some more details:

- System: up-to-date Ubuntu Focal, gnome-shell 3.37.0
- This is tested on a Thinkpad T480s, Intel graphics, DisplayPort connection (DP over USB-C)
- Kernel 5.0.0-7-generic
- Happens with both Wayland and Xorg
- Happens even if I create a new user on my system, with a clean home directory

I already experienced this problem with Disco, but then it somehow went away. Now it's back with Focal. Upstream bug (filed back when I was using Disco):

https://gitlab.gnome.org/GNOME/gnome-shell/issues/1046