I'm running Gnome (Wayland) on Arch Linux. I have configured the environment not to go to suspend, etc. when the lid is closed. When I close the lid, the monitor turns off for a second or two (as though it's responding to the laptop lid state changing), then turns back on. Likewise, when I open the lid, it turns off for a second or two, then turns back on. (If I wasn't careful, I might have thought the monitor was turning off when closing the lid...sneaky Gnome!)
How do I make it so that the computer stays on and responds to, e.g. network events, but the monitor doesn't stay on all day?