Hi, Did you get this laptop "bare" or did it come with an MSWin install? If so and if you kept it, the 1st step would be to boot under that OS and see how things behave then. Dan is right about checking the hardware key but in my experience with laptops from at least the past 10 years is that it gives you a shortcut to modify whatever you configured as the default via software. I have an almost 10y old Acer netbook (now) under Win10 that I've configured to send only audio to the AV amp when I connect that device via HDMI, i.e. I disabled sending image data. I didn't find a way to do that under KDE4/Plasma so there it's set to screen mirroring, using xrandr to add the weirdish 1366x768 resolution to the display that my AV amp does NOT have ;)
That said, I've seen remarks before about multi-head issues under KF5/Plasma, IIRC cited as a reason to move to using Wayland instead of X11. I cannot comment on that (other than noting that my mouse starts moving erratically when I use an external screen under KDE4/Plasma). Other ideas: check your X11 server settings and verify that you have the required Xinerama-related packages installed. Try another desktop environment and if things work better there, try its window manager under KDE (`foowm --replace` should do the trick for testing). About the TV's resolution: I'd advise you run it at its native resolution, meaning put the laptop at its resolution when you start watching movies on the TV (or configure the TV as an additional screen instead of a mirroring device). The software you watch your movie in will have good quality scaling to map the video's resolution to your screen resolution and if you're streaming you can probably pick a resolution that's appropriate for your TV. Forcing your TV to run at a higher resolution than what its panel has to offer will lead to a loss of quality because it will use a very basic scaling algorithm. R
