Add layer_dismiss module: a key that drops the layer that delivered it

[dscarpetti]

What

KC_LAYER_DISMISS (alias LY_DSMS) is an assignable key that, on release, turns off whichever layer resolved the keypress, and emits nothing of its own.

It is the missing third layer primitive. TG() raises and lowers one hardcoded layer. Layer Lock holds the current layer on. TO(0) exits but tears down every other active layer with it. There was no way to bind "let this layer go", the lowering half of a toggle, aimed at the layer the key came from, so a single binding works on any layer it is placed on. On the base layer the key is a deliberate no-op so the root cannot be dismissed.

Why (the automouse case)

Automouse currently offers timeout-based deactivation or oneshot tap-to-exit, and neither fits a common want: mouse keys should keep the layer alive, while chosen positions dismiss it. With this keycode that policy moves into the keymap. Bind KC_LAYER_DISMISS on whichever automouse-layer positions should exit, and each becomes a consumed tap-to-wake: the layer drops, nothing types, and the next keystroke lands on the layout below. The automouse module needs no changes, it already tolerates its layer being turned off externally and re-raises on the next motion.

Dropping on release keeps rollover safe. A fast second keystroke arrives while the layer is still up and resolves through its transparency to the layout below, so nothing is lost when typing immediately after a wake tap.

Scope

Tiny footprint, large possibility space, predictable behavior: one keycode, one hook, about 30 lines including comments. Generic by construction: any transient layer can carry dismiss positions, and per-position policy lives in keymap data rather than firmware configuration.

Tested on a Voyager with navigator_trackball + automouse (three dismiss positions on the mouse layer, built against firmware25 with this module added)

[dscarpetti]

Use surfaced something worth mentioning, there is a hold-and-release behavior that comes along for free.

Tap dismisses as designed. What I had not planned is what a hold does when composed with automouse. While the key is held, automouse counts it as layer activity and keeps refreshing its timer, so the layer stays open indefinitely through any pause. Release then drops it instantly. The dismiss key is thereby also a sustain key: rest a finger on it and pointer mode persists, lift it and you are typing again. My layout previously dedicated an entire second held layer to exactly that persistence job, and this made the layer deletable.

This falls out of dropping on release rather than press, which the PR justified for rollover safety.

One important ordering note is that the sustain behavior requires automouse to be listed before layer_dismiss in the keymap's modules list, so that automouse sees the held key's press before layer_dismiss consumes it. Otherwise, tap-to-dismiss still works but a hold no longer counts as activity.

I'm now running with a short automouse idle timeout, which gives me time for double clicks. When doing sustained mousing, I hold down a dismiss key to keep the mouse live and then I pop directly back into typing mode when I release it.