I made a simple script that I'm working on to work around this issue. It uses evtest to read mouse input. But be aware that this may interfere with other shortcuts used by other software (backward / forward in Firefox for example).

#!/bin/bash

MOUSE="/dev/input/event6" 
KEYBOARD="/dev/input/event4"
MOD_STATE_FILE="/tmp/mod_pressed_state"  

echo "false" > "$MOD_STATE_FILE"


evtest "$KEYBOARD" | while read -r line; do
    if [[ "$line" == *"code 125"* && "$line" == *"value 1"* ]]; then
        echo "true" > "$MOD_STATE_FILE"  
        echo "Mod pressed"
    fi

    if [[ "$line" == *"code 125"* && "$line" == *"value 0"* ]]; then
        echo "false" > "$MOD_STATE_FILE"  
        echo "Mod not pressed"
    fi
done &


evtest "$MOUSE" | while read -r line; do
    mod_pressed=$(cat "$MOD_STATE_FILE")

    if [[ "$mod_pressed" == "true" && "$line" == *"code 275"* && "$line" == *"value 1"* ]]; then
        niri msg action focus-column-left
    fi

    if [[ "$mod_pressed" == "true" && "$line" == *"code 276"* && "$line" == *"value 1"* ]]; then
        niri msg action focus-column-right
    fi
done

I also need this as I just got a mouse with these two buttons. Is it a simple change? @YaLTeR

It is not a very simple change, a lot of types there are hardcoded for keyboard.

I guess @Paul-Eau's workaround looks good then until this gets done properly.

Edit: I just tested it and it causes firefox to go back and forth in the tab history. Any suggestions to solve this with/without grabbing the mouse exclusively?

Edit: I just tested it and it causes firefox to go back and forth in the tab history. Any suggestions to solve this with/without grabbing the mouse exclusively?

In about:config set mousebutton.4th.enabled and mousebutton.5th.enabled to false.

Can https://wiki.archlinux.org/title/Interception-tools#Combine_devices be useful?

If your mouse supports libratbag (which you can use either through ratbagctl or piper) you can also use button remapping as a workaround