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).

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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.