How to set screen borders for HDMI output

What you are looking at is a"feature called "overscan" (or "zoom" on some TVs). It's the TV itself cutting off the edges!

So, look for a feature called overscan (or zoom) in the setup of the TV, and disable it!

Some history:

Long time ago, when cathode ray tubes - CRTs - were used as television displays, and the images were transferred in an analogue way, it had a technical reason, it was useful.
The TV stations were used to always have a border with nothing important in it.

With pixel matrix displays like LCD, overscan no longer makes sense.
But people are used to have it, and think it's better to have the feature than not have it.

If you want to sell LCD TVs, you have customers asking for it, and because it costs nothing to provide it, you better list that feature in the description.
It will not do any harm, no problem. Except if someone enables it.

My TV also does that. Usually I just run

xrandr --output LVDS1 --primary --auto --output HDMI1 --auto --same-as LVDS1

This fixes the wrong resolution on the monitor, making it a mirror of my laptop.

I have this command conveniently aliased to something I can remember:

alias xrandr-hdmi-mirror="xrandr --output LVDS1 --primary --auto --output HDMI1 --auto --same-as LVDS1"

This way I just run xrandr-hdmi-mirror whenever I connect my TV.

Of course, you should adjust HDMI1 and/or LVDS1 to your system. Run xrandr to discover which device you should use.

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Updated answer

Since you have only one monitor (your TV) try the following:

1. Discover the output name of your TV. It is probably HDMI1 or VGA1. To do it, run xrandr then search for the word "connected". For example, if you see HDM1 connected, then your TV is HDMI1.

2. Try doing a xrandr --output HDMI1 --auto. See if the resolution gets okay.

3. If not, you'll have to do a xrandr --output HDMI1 --mode 1024x768, for example. However, you should replace 1024x768 to a resolution that your TV supports. The output of xrandr will tell you the available ones.

I had the same problem with my Samsung TV connecting to my laptop via HDMI and found the solution by going to the TV Menu > Picture > Screen Adjustment > chose "screen fit" that fixed it. There you will find other options too but for me the screen fit did the trick and that over scan issue was fixed.

All this time i was thinking it was the display setting on the Ubuntu laptop i needed to fix. Hope This helps other.