1. Find the ID

of your touchpad with xinput list:

$ xinput list
⎡ Virtual core pointer                      id=2    [master pointer  (3)]
⎜   ↳ Virtual core XTEST pointer               id=4    [slave  pointer  (2)]
⎜   ↳ SynPS/2 Synaptics TouchPad               id=11   [slave  pointer  (2)]
⎜   ↳ TPPS/2 IBM TrackPoint                    id=12   [slave  pointer  (2)]
⎣ Virtual core keyboard                     id=3    [master keyboard (2)]
    ↳ Virtual core XTEST keyboard              id=5    [slave  keyboard (3)]
    ↳ Power Button                             id=6    [slave  keyboard (3)]
    ↳ Video Bus                                id=7    [slave  keyboard (3)]
    ↳ Sleep Button                             id=8    [slave  keyboard (3)]
    ↳ Integrated Camera: Integrated C          id=9    [slave  keyboard (3)]
    ↳ AT Translated Set 2 keyboard             id=10   [slave  keyboard (3)]
    ↳ ThinkPad Extra Buttons                   id=13   [slave  keyboard (3)]

Apparently, the one called „… TouchPad“ has id=11.

2. Get the buttom map

with the ID:

xinput get-button-map 11
1 2 3 4 5 6 7 

3. Try to set a new button map

Now you have to use trial and error to figure out the meaning of the button map. You can typically assume that 1 is the left-click-action. The first thing to try is to guess that the second position is the bottom middle button. Replace it with left click action (1):

$ xinput set-button-map 11 1 1 3 4 5 6 7
                             ^

4. Check that it works

Thanks to Adam’s answer (which inspired this one) I only needed one try to get the result I wanted:

+-----------------+
|                 |
|                 |
|                 |
|                 |
|11111 11111 33333|
|11111 11111 33333|
+-----------------+

That is, the left two thirds of the bottom stripe act as the left button while the right third acts as the right button.

5. What if it doesn’t work?

Try replacing other parts of the button map with the key 1 similar to step 3. See how the touchpad reacts. If you can’t get the desired result, reset the button map to what you obtained in step 1, to get back the original behaviour.

https://unix.stackexchange.com/questions/438725/disabling-middle-click-on-bottom-of-a-clickpad-touchpad

https://docs.xfce.org/xfce/xfce4-settings/mouse