Feature: Support binding actions to a single modifier key tap (e.g., Super key to open launcher)

[Rejzi-dich]

Currently, Niri does not allow binding an action to a single modifier key (like Mod / Super). Attempting to use Mod in the config results in an "invalid key" error.

This is a very common and highly requested UX pattern for users coming from KDE Plasma, GNOME, where tapping the Super/Cmd/Win key opens the app launcher or overview.

Currently, the only way to achieve this in Niri is via third-party tools like keyd, which remaps a quick tap of the Super key to an unused keycode (like F13). While this works, it requires external daemon setup and is not intuitive.

It would be great if Niri natively supported a "tap" behavior for modifier keys, distinguishing between:

A quick tap and release (triggers an action, like opening the launcher).
A hold (acts as a standard modifier for keybinds like Mod+Enter).

A possible syntax could be something like:

Mod tap=true { spawn "rofi" "-show" "run"; }

Or treating a "press and release without other keys" as a valid bind.

[neunato]

There is #3621