Most stuff is handled by Noctalia Shell. My configuration sort of assumes you have KDE plasma installed already, otherwise you have to install a handful of utilities individually. Stuff I had to fix:

Discord big cursor

environment {
    ELECTRON_OZONE_PLATFORM_HINT "auto"
    ELECTRON_OZONE_PLATFORM "wayland;xcb"
}

Keyboard layouts + compose key

        xkb {
            // You can set rules, model, layout, variant and options.
            // For more information, see xkeyboard-config(7).
            layout "us,us"
            variant "colemak_dh,"
            options "grp:win_space_toggle,compose:ralt,lv5:rctrl_switch"
        }

Dolphin is broken and can't find default applications

(Not Really) A Guide To Setup Dolphin & KDE File Picker On Niri - Arch Linux

OBS can't see my screen

"It doesn't work" Troubleshooting Checklist · emersion/xdg-desktop-portal-wlr Wiki

Add this to ~/.config/xdg-desktop-portal/niri-portals.conf (needs the respective xdg-desktop-portal-* packages)

[preferred]
default=kde;gnome;gtk;
org.freedesktop.impl.portal.Access=kde;gtk;
org.freedesktop.impl.portal.ScreenCast=gnome
org.freedesktop.impl.portal.Screenshot=gnome
org.freedesktop.impl.portal.Notification=kde;gtk;
org.freedesktop.impl.portal.Secret=gnome-keyring;

This also fixes firefox not using the KDE file picker. Might need to set widget.use-xdg-desktop-portal.file-picker in about:config (firefox) to 2.

Password prompts don't work

Added this to my niri config:

spawn-at-startup "/usr/lib/polkit-kde-authentication-agent-1"

(can substitute the polkit agent by a different one depending on what you want to use, as per https://wiki.archlinux.org/title/Polkit)

Steam has a different cursor from everything else

To solve this and also otherwise tweak gtk stuff look and feel: install nwg-look and tweak the settings.

Configuring sleep behaviour

Using swayidle with a basic configuration at ~/.config/swayidle/config. Also add spawn-at-startup swayidle to your niri config.

timeout 300 "niri msg action power-off-monitors"
timeout 900 "systemctl suspend"
before-sleep "qs -c noctalia-shell ipc call lockScreen lock"

Inhibit idle/sleep when audio is playing (useful for games)

if-audio-inhibit-idle (somewhere in your path)

#!/usr/bin/env fish

function get_active_streams
    # get stream info, join each 3 lines into a single stream info, grep the active ones
    wpctl status | rg -U --multiline-dotall "Streams:\n(.+?\n\n)" -or '$1' | paste -d' ' - - - | rg "\d+\. (\S+).+\[active\]" -or '$1'
end

set ignore_list "Solanum"

function check_audio
    for s in (get_active_streams)
        if not contains $s $ignore_list
            return 0
        end
    end
    return 1
end

while true
    if check_audio;
        # we use idle instead of sleep because I don't want the screen to turn off either
        systemd-inhibit --what=idle --why="Audio playback active" --mode=block sleep infinity &
        set inhibit_pid $last_pid
        while check_audio;
            sleep 5
        end
        kill $inhibit_pid
        wait $inhibit_pid
    end
    sleep 5
end

then add this to your niri config: spawn-at-startup "if-audio-inhibit-idle"