Underscan slider missing in Display set-up window

I found a solution for those who have low-quality TVs which, like my own, may not allow you to turn off overscan in their own settings.

While the slider is gone Displays preference panel GUI, the preference itself is still accessible and editable in a file. It's stored in /private/var/db/.com.apple.iokit.graphics, and you can edit it as long as you have administrator privileges. I, personally, opened it on vi (because TextEdit was refusing to open at all with sudo, something which I really don't understand because it used to work fine in older versions of macOS), by opening Terminal and inputting the command:

sudo vi /private/var/db/.com.apple.iokit.graphics

Then it was only a matter of editing the value for the property “pscn” (to start editing text in vi like in a normal GUI text editor, just press A and use your arrow keys to change your insertion point – or vice-versa, it doesn't really matter which one you do first), which will appear like this:

<key>pscn</key>

<integer>10000</integer>

By the way, I'm not sure whether these were a leftover from back when I still had the slider, but if they aren't there, you can always try to add them after these other two, like so:

<key>framebuffer-rotation</key>

<integer>0</integer>

<key>pscn</key>

<integer>10000</integer>

Please bear in mind that you will have at least two monitors in that file (your internal one and the one you're trying to adjust), and maybe even more (in my case, I also had what was probably a leftover from a projector I connected my Mac into for some lecture presentations I gave at my Uni), so if none of them has that “pscn” tag present, you may have to figure it out by trial and error (and since the rest of the tags aren't exactly human-readable – seriously; you'd expect to see some brand names and models, but it's mostly gibberish – I have no idea whether you can underscan your internal monitor without ill effects, so do it at your own risk).

Anyway, I digress… “10000” is the baseline value, meaning it won't be underscanned or overscanned. Any value under that (in my TV's case, even though I didn't get it pixel-perfect and it still looks a bit analogue video-ish, I eventually figured it was around 9530) will underscan the output and, conversely, any value above it will overscan it even further.

So, just try a value, exit vi while saving the file by pressing the Escape key, followed by semi-colon, X, and Return/Enter. Reboot, see how it turned out, and if you didn't nail the value right away, give it a few more tries until you narrow it down.

Oh, and finally, I'm not sure if this is really necessary, but I believe that at my first try the changes I did to the file didn't really stick after rebooting. I changed that file's permissions to be read-only for all users (including “system”) before rebooting, and changed it back to read & write everytime I had to edit it again. Eventually, I stopped bothering about changing it back and forth between reboots, and the new values I was trying were always sticking after all.