When an external monitor is connected, gnome-settings-daemon adds a systemd inhibitor to block handle-lid-switch events:

$ systemd-inhibit --list --mode=block
     Who: toupeira (UID 1000/toupeira, PID 2021/gsd-power)
    What: handle-lid-switch
     Why: Multiple displays attached
    Mode: block

I currently don't have an external keyboard for my laptop, so this behaviour is really unexpected and not usable. It would be great to make this configurable, or maybe smarter to check for external inputs as well.

Some findings:

• Ubuntu patches in a setting for this called lid-close-suspend-with-external-monitor (link to the actual patch is dead)
• gnome-tweaks confusingly has a setting called "Suspend when laptop lid is closed", but even when enabled this doesn't override the gsd-power inhibitor
• logind has a setting LidSwitchIgnoreInhibited, but this doesn't apply to low-level locks such as handle-lid-switch