Can't change brightness, get 'No external display' warning

I’ve just download Leap, and I’ve come across an issue.

I can’t seem to change my brightness. When I press the keys, nothing happens; when I drag the slider from the power icon in the system tray, this warning shows up:

https://imgur.com/a/K4TDM8U

The warning also pops up when I press f4 (which didn’t happen in Tumbleweed), and periodically for no apparent reason. I’ve been able to find one thread that mentioned this issue, but it only addressed how to stop the warning from showing, not what caused it.

You are talking about all sorts of keys you press (without explaining which ones) and about using a slider. That gives me the idea that you use some GUI and thus probably some Desktop Environment. For you forgot to explain what DE you use, KDE, Gnome, …

Remember that people can not look over your shoulder and thus only can be sure what you do and see from your description, not from assumption.

Yeah, yeah, sorry. I wrote that kind of rushedly.

I’m using KDE.

The keys which I normally use to change brightness are f2 and f3 (to lower and increase, respectively).

f4 controls how external displays are shown (e.g. Both monitor and external display have the same display, only external display appears, or the external display is an extension of the monitor, etc).

The slider appears when you click the battery/power management icon in the system tray.

Again, sorry about that.