[Core] Add Layer Lock feature (#23430)

Co-authored-by: Daniel <1767914+iamdanielv@users.noreply.github.com>
Co-authored-by: Pascal Getreuer <getreuer@google.com>
Co-authored-by: Pascal Getreuer <50221757+getreuer@users.noreply.github.com>

docs/features/layer_lock.md

@@ -0,0 +1,139 @@
+# Layer Lock
+
+Some [layer switches](../feature_layers#switching-and-toggling-layers) access
+the layer by holding the key, including momentary layer `MO(layer)` and layer
+tap `LT(layer, key)` keys. You may sometimes need to stay on the layer for a
+long period of time. Layer Lock "locks" the current layer to stay on, supposing
+it was accessed by one of:
+
+ * `MO(layer)` momentary layer switch
+ * `LT(layer, key)` layer tap
+ * `OSL(layer)` one-shot layer
+ * `TT(layer)` layer tap toggle
+ * `LM(layer, mod)` layer-mod key (the layer is locked, but not the mods)
+
+Press the Layer Lock key again to unlock the layer. Additionally, when a layer
+is locked, layer switch keys that turn off the layer such as `TO(other_layer)`
+will unlock it.
+
+
+## How do I enable Layer Lock
+
+In your rules.mk, add:
+
+```make
+LAYER_LOCK_ENABLE = yes
+```
+
+Pick a key in your keymap on a layer you intend to lock, and assign it the
+keycode `QK_LAYER_LOCK` (short alias `QK_LLCK`). Note that locking the base
+layer has no effect, so typically, this key is used on layers above the base
+layer.
+
+
+## Example use
+
+Consider a keymap with the following base layer.
+
+![Base layer with a MO(NAV) key.](https://i.imgur.com/DkEhj9x.png)
+
+The highlighted key is a momentary layer switch `MO(NAV)`. Holding it accesses a
+navigation layer.
+
+![Nav layer with a Layer Lock key.](https://i.imgur.com/2wUZNWk.png)
+
+
+Holding the NAV key is fine for brief use, but awkward to continue holding when
+using navigation functions continuously. The Layer Lock key comes to the rescue:
+
+1. Hold the NAV key, activating the navigation layer.
+2. Tap Layer Lock.
+3. Release NAV. The navigation layer stays on.
+4. Make use of the arrow keys, etc.
+5. Tap Layer Lock or NAV again to turn the navigation layer back off.
+
+A variation that would also work is to put the Layer Lock key on the base layer
+and make other layers transparent (`KC_TRNS`) in that position. Pressing the
+Layer Lock key locks (or unlocks) the highest active layer, regardless of which
+layer the Layer Lock key is on.
+
+
+## Idle timeout
+
+Optionally, Layer Lock may be configured to unlock if the keyboard is idle
+for some time. In config.h, define `LAYER_LOCK_IDLE_TIMEOUT` in units of
+milliseconds:
+
+```c
+#define LAYER_LOCK_IDLE_TIMEOUT 60000  // Turn off after 60 seconds.
+```
+
+
+## Functions
+
+Use the following functions to query and manipulate the layer lock state.
+
+| Function                   | Description                        |
+|----------------------------|------------------------------------|
+| `is_layer_locked(layer)`   | Checks whether `layer` is locked.  |
+| `layer_lock_on(layer)`     | Locks and turns on `layer`.        |
+| `layer_lock_off(layer)`    | Unlocks and turns off `layer`.     |
+| `layer_lock_invert(layer)` | Toggles whether `layer` is locked. |
+
+
+## Representing the current Layer Lock state
+
+There is an optional callback `layer_lock_set_user()` that gets called when a
+layer is locked or unlocked. This is useful to represent the current lock state
+for instance by setting an LED. In keymap.c, define
+
+```c
+bool layer_lock_set_user(layer_state_t locked_layers) {
+  // Do something like `set_led(is_layer_locked(NAV));`
+  return true;
+}
+```
+
+The argument `locked_layers` is a bitfield in which the kth bit is on if the kth
+layer is locked. Alternatively, you can use `is_layer_locked(layer)` to check if
+a given layer is locked.
+
+
+## Combine Layer Lock with a mod-tap
+
+It is possible to create a [mod-tap MT key](../mod_tap) that acts as a modifier
+on hold and Layer Lock on tap. Since Layer Lock is not a [basic
+keycode](../keycodes_basic), attempting `MT(mod, QK_LLCK)` is invalid does not
+work directly, yet this effect can be achieved through [changing the tap
+function](../mod_tap#changing-tap-function). For example, the following
+implements a `SFTLLCK` key that acts as Shift on hold and Layer Lock on tap:
+
+```c
+#define SFTLLCK LSFT_T(KC_0)
+
+// Use SFTLLCK in your keymap...
+
+bool process_record_user(uint16_t keycode, keyrecord_t *record) {
+    switch (keycode) {
+        case SFTLLCK:
+            if (record->tap.count) {
+                if (record->event.pressed) {
+                    // Toggle the lock on the highest layer.
+                    layer_lock_invert(get_highest_layer(layer_state));
+                }
+                return false;
+            }
+            break;
+
+        // Other macros...
+    }
+    return true;
+}
+```
+
+In the above, `KC_0` is an arbitrary placeholder for the tapping keycode. This
+keycode will never be sent, so any basic keycode will do. In
+`process_record_user()`, the tap press event is changed to toggle the lock on
+the highest layer. Layer Lock can be combined with a [layer-tap LT
+key](../feature_layers#switching-and-toggling-layers) similarly.
+