"Restore", which is found in the Windows section in Settings - Keyboard shortcuts, is not only assigned to Super+Down, but also to the "traditional" shortcut Alt+F5. You can disable the binding to Alt+F5 and retain the Super+Down binding:
- Go to "Settings" - "Keyboard"
- Click "View and customize shortcuts" in the right pane
- Under the section "Windows", click "Restore window".
- The "Set shortcut" dialog appears.
- Press Super+Down: this will set that key only.
gsettings in the terminal:
gsettings set org.gnome.desktop.wm.keybindings unmaximize "['<Super>Down']"
That removes the assignment to Alt+F5, but maintains Super+Down for unmaximizing a window. Reset to the default (
['<Super>Down', '<Alt>F5']) with the command
gsettings reset org.gnome.desktop.wm.keybindings unmaximize
You can edit this and many other "hidden" settings using
dconf-editor, not installed by default.