[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>

quantum/process_keycode/process_layer_lock.h

@@ -0,0 +1,69 @@
+// Copyright 2022-2023 Google LLC
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+/**
+ * @file layer_lock.h
+ * @brief Layer Lock, a key to stay in the current layer.
+ *
+ * Overview
+ * --------
+ *
+ * Layers are often accessed by holding a button, e.g. with a momentary layer
+ * switch `MO(layer)` or layer tap `LT(layer, key)` key. But you may sometimes
+ * want to "lock" or "toggle" the layer so that it stays on without having to
+ * hold down a button. One way to do that is with a tap-toggle `TT` layer key,
+ * but here is an alternative.
+ *
+ * This library implements a "Layer Lock key". When tapped, it "locks" the
+ * highest layer to stay active, assuming the layer was activated by one of the
+ * following keys:
+ *
+ *  * `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)
+ *
+ * Tapping the Layer Lock key again unlocks and turns off the layer.
+ *
+ * @note When a layer is "locked", other layer keys such as `TO(layer)` or
+ * manually calling `layer_off(layer)` will override and unlock the layer.
+ *
+ * Configuration
+ * -------------
+ *
+ * Optionally, a timeout may be defined so that Layer Lock disables
+ * automatically if not keys are pressed for `LAYER_LOCK_IDLE_TIMEOUT`
+ * milliseconds. Define `LAYER_LOCK_IDLE_TIMEOUT` in your config.h, for instance
+ *
+ *     #define LAYER_LOCK_IDLE_TIMEOUT 60000  // Turn off after 60 seconds.
+ *
+ * and call `layer_lock_task()` from your `matrix_scan_user()` in keymap.c:
+ *
+ *     void matrix_scan_user(void) {
+ *       layer_lock_task();
+ *       // Other tasks...
+ *     }
+ *
+ * For full documentation, see
+ * <https://getreuer.info/posts/keyboards/layer-lock>
+ */
+
+#pragma once
+
+#include <stdint.h>
+#include <stdbool.h>
+#include "action.h"
+
+bool process_layer_lock(uint16_t keycode, keyrecord_t* record);