gsettings get org.gnome.gnome-system-monitor window-state
gsettings get org.gnome.gnome-system-monitor maximized
But both of these seem to get updated every time you close the system monitor window. So after you have already recovered the window to a usable position with mouse, or cursors, or anyhow, there is no point in setting these; they will just get updated again.
With that said, here is a command capable of restoring the window; from the above it can be seen that it has to be issued while there is no window open for system monitor:
gsettings set org.gnome.gnome-system-monitor window-state '(1000,800,50,50)'
See more info about window-positions here.
Other ways of recovering:
Holding down a modifier key during click-dragging makes the entire surface of the window available for click-drag (not just the header).
This modifier key used to be
Alt on older releases, and left
Super (the left-side "Windows" key) on recent releases. Try both.
I believe a few window-related keyboard shortcuts are enabled by default on 20.04.
So whenever a window is focused, a modifier key + left or right arrow will move the window to the corresponding side of the screen(s) theoretically (hopefully?) in such way that the header remains visible. The modifier + up arrow will make the window maximized.
This modifier on 20.04 by default is the left
Super key again (left "Windows" key.)
These are mostly kept in the
gsettings API, but for an easier management, graphic config apps are available.
dconf-editor app will reveal the window-manager's settings in the
org.gnome.mutter.keybindings schema. It's easy to spot the window-positioning shortcuts.
I haven't found the setting for the full-window click-dragging modifier key in this config. For that I found the
gnome-tweaks app: it has a "Windows" sidebar item, and on the corresponding panel a "Window Action Key" setting.