[Left Alt]+Tab doesn't work or How to change a Level3 Shift to a normal ALT?

Try xev and xmodmap:

Run the command xev then hover your mouse over the black square.
Press the Alt key, and make a note of the following: My output was this (shortened): KeyPress event... keycode 64 (keysym 0xffe9, Alt_L), ... XFilterEvent returns: False
The important part is the keycode: Mine was 64, but yours is probably different.
Run the command xmodmap -e "keycode 64 = Alt_L" substituting 64 for whatever output you get (possibly 135 on your keyboard)
This should have changed it. If it has, you then need to run the following command: xmodmap -pke >~/.Xmodmap
Create a file called .xinitrc in your home folder (/home/username)
In that file paste the line xmodmap .Xmodmap. This should save it forever.

See this answer for details about remapping.

My pronouns are He / Him

Both of these solutions were a bit too hacky for my taste, so I dug a little deeper. The solution I found only uses dconf-editor.

Just open dconf-editor and search (ctrl+F) for xkb-options and update the value. In my case, the value associated with the xkb-options key was

['terminate:ctrl_alt_bksp','lv3:alt_switch']

The option 'lv3:alt_switch' was the one that was causing my headache. I simply changed the value to:

['terminate:ctrl_alt_bksp']

That's it!

Check current layout defaults

Check used layout/variant names, example my case:

$ setxkbmap -query

rules:      evdev
model:      pc105
layout:     us,us
variant:    altgr-intl

Open the file in /usr/share/X11/xkb/symbols/ with same name of layout.
```
sudo nano /usr/share/X11/xkb/symbols/us
```

Scroll down to the exact variant

xkb_symbols "altgr-intl" {

 name[Group1]= "English (international AltGr dead keys)";

...

include "level3(ralt_switch)"
};

See if you have include "level3(ralt_switch)" (or maybe include "level3(lalt_switch)", not sure if that's possible), if so remove its line.
Run to update layout
```
sudo dpkg-reconfigure xkb-data
```

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