How do I move child windows without moving or minimizing parent in GNOME 3?
It can be done using '(GNOME) Tweaks' (previously known as '(GNOME) Tweak Tool'). There should be a toggle button for "Attach Modal Dialogues" in the Windows section. Disable it.
To install Tweaks, run
sudo apt install gnome-tweaks
(or sudo apt install gnome-tweak-tool
).
Alternative solution:
Open Terminal and run
gsettings set org.gnome.shell.overrides attach-modal-dialogs false
This detaches modal dialogues from the parent window and hence should fix your problem.
If this doesn't work you need to toggle another gsettings
/dconf
key following OP's answer or use (GNOME) Tweaks as mentioned above.
Thanks to pomsky's suggestion above, which doesn't quite do the job but led me to the switch I needed.
I used dconf-editor to turn off /org/gnome/mutter/attach-modal-dialogs.
For what its worth the minimizing of the libreoffice spreadsheet appears to have been a side-effect of the parent window being maximized.
Update
Pomsky's suggestion of using the tweak tool is much better than using dconf-editor.