[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>

builddefs/generic_features.mk

@@ -36,6 +36,7 @@ GENERIC_FEATURES = \
     HAPTIC \
     KEY_LOCK \
     KEY_OVERRIDE \
+    LAYER_LOCK \
     LEADER \
     MAGIC \
     MOUSEKEY \

data/constants/keycodes/keycodes_0.0.6_quantum.hjson

@@ -3,5 +3,12 @@
         "0x7C20": "!delete!", // old QK_OUTPUT_AUTO
         "0x7C21": "!delete!", // old QK_OUTPUT_USB
         "0x7C22": "!delete!", // old QK_OUTPUT_BLUETOOTH
+        "0x7C7B": {
+            "group": "quantum",
+            "key": "QK_LAYER_LOCK",
+            "aliases": [
+                "QK_LLCK"
+            ]
+        }
     }
 }

data/mappings/info_config.hjson

@@ -64,6 +64,9 @@
     "WEAR_LEVELING_BACKING_SIZE": {"info_key": "eeprom.wear_leveling.backing_size", "value_type": "int", "to_json": false},
     "WEAR_LEVELING_LOGICAL_SIZE": {"info_key": "eeprom.wear_leveling.logical_size", "value_type": "int", "to_json": false},
 
+    // Layer locking
+    "LAYER_LOCK_IDLE_TIMEOUT": {"info_key": "layer_lock.timeout", "value_type": "int"},
+
     // Indicators
     "LED_CAPS_LOCK_PIN": {"info_key": "indicators.caps_lock"},
     "LED_NUM_LOCK_PIN": {"info_key": "indicators.num_lock"},

data/schemas/keyboard.jsonschema

@@ -375,6 +375,12 @@
             }
         },
         "keycodes": {"$ref": "qmk.definitions.v1#/keycode_decl_array"},
+        "layer_lock": {
+            "type": "object",
+            "properties": {
+                "timeout": {"$ref": "qmk.definitions.v1#/unsigned_int"}
+            }
+        },
         "layout_aliases": {
             "type": "object",
             "additionalProperties": {"$ref": "qmk.definitions.v1#/layout_macro"}

docs/_sidebar.json

@@ -123,6 +123,7 @@
                     { "text": "Key Lock", "link": "/features/key_lock" },
                     { "text": "Key Overrides", "link": "/features/key_overrides" },
                     { "text": "Layers", "link": "/feature_layers" },
+                    { "text": "Layer Lock", "link": "/features/layer_lock" },
                     { "text": "One Shot Keys", "link": "/one_shot_keys" },
                     { "text": "OS Detection", "link": "/features/os_detection" },
                     { "text": "Raw HID", "link": "/features/rawhid" },

docs/feature_layers.md

@@ -17,6 +17,9 @@ These functions allow you to activate layers in various ways. Note that layers a
 * `TO(layer)` - activates *layer* and de-activates all other layers (except your default layer). This function is special, because instead of just adding/removing one layer to your active layer stack, it will completely replace your current active layers, uniquely allowing you to replace higher layers with a lower one. This is activated on keydown (as soon as the key is pressed).
 * `TT(layer)` - Layer Tap-Toggle. If you hold the key down, *layer* is activated, and then is de-activated when you let go (like `MO`). If you repeatedly tap it, the layer will be toggled on or off (like `TG`). It needs 5 taps by default, but you can change this by defining `TAPPING_TOGGLE` -- for example, `#define TAPPING_TOGGLE 2` to toggle on just two taps.
 
+See also the [Layer Lock key](features/layer_lock), which locks the highest
+active layer until pressed again.
+
 ### Caveats {#caveats}
 
 Currently, the `layer` argument of `LT()` is limited to layers 0-15, and the `kc` argument to the [Basic Keycode set](keycodes_basic), meaning you can't use keycodes like `LCTL()`, `KC_TILD`, or anything greater than `0xFF`. This is because QMK uses 16-bit keycodes, of which 4 bits are used for the function identifier and 4 bits for the layer, leaving only 8 bits for the keycode.

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.
+

docs/keycodes.md

@@ -387,6 +387,14 @@ See also: [Key Lock](features/key_lock)
 |---------|--------------------------------------------------------------|
 |`QK_LOCK`|Hold down the next key pressed, until the key is pressed again|
 
+## Layer Lock {#layer-lock}
+
+See also: [Layer Lock](features/layer_lock)
+
+|Key            |Aliases  |Description                       |
+|---------------|---------|----------------------------------|
+|`QK_LAYER_LOCK`|`QK_LLCK`|Locks or unlocks the highest layer|
+
 ## Layer Switching {#layer-switching}
 
 See also: [Layer Switching](feature_layers#switching-and-toggling-layers)

quantum/keyboard.c

@@ -140,6 +140,9 @@ along with this program.  If not, see <http://www.gnu.org/licenses/>.
 #ifdef OS_DETECTION_ENABLE
 #    include "os_detection.h"
 #endif
+#if defined(LAYER_LOCK_ENABLE) && LAYER_LOCK_IDLE_TIMEOUT > 0
+#    include "layer_lock.h"
+#endif // LAYER_LOCK_ENABLE
 
 static uint32_t last_input_modification_time = 0;
 uint32_t        last_input_activity_time(void) {
@@ -655,6 +658,10 @@ void quantum_task(void) {
 #ifdef SECURE_ENABLE
     secure_task();
 #endif
+
+#if defined(LAYER_LOCK_ENABLE) && LAYER_LOCK_IDLE_TIMEOUT > 0
+    layer_lock_task();
+#endif
 }
 
 /** \brief Main task that is repeatedly called as fast as possible. */

quantum/keycodes.h

@@ -759,6 +759,7 @@ enum qk_keycode_defines {
     QK_TRI_LAYER_UPPER = 0x7C78,
     QK_REPEAT_KEY = 0x7C79,
     QK_ALT_REPEAT_KEY = 0x7C7A,
+    QK_LAYER_LOCK = 0x7C7B,
     QK_KB_0 = 0x7E00,
     QK_KB_1 = 0x7E01,
     QK_KB_2 = 0x7E02,
@@ -1445,6 +1446,7 @@ enum qk_keycode_defines {
     TL_UPPR    = QK_TRI_LAYER_UPPER,
     QK_REP     = QK_REPEAT_KEY,
     QK_AREP    = QK_ALT_REPEAT_KEY,
+    QK_LLCK    = QK_LAYER_LOCK,
 };
 
 // Range Helpers
@@ -1501,7 +1503,7 @@ enum qk_keycode_defines {
 #define IS_UNDERGLOW_KEYCODE(code) ((code) >= QK_UNDERGLOW_TOGGLE && (code) <= QK_UNDERGLOW_SPEED_DOWN)
 #define IS_RGB_KEYCODE(code) ((code) >= RGB_MODE_PLAIN && (code) <= RGB_MODE_TWINKLE)
 #define IS_RGB_MATRIX_KEYCODE(code) ((code) >= QK_RGB_MATRIX_ON && (code) <= QK_RGB_MATRIX_SPEED_DOWN)
-#define IS_QUANTUM_KEYCODE(code) ((code) >= QK_BOOTLOADER && (code) <= QK_ALT_REPEAT_KEY)
+#define IS_QUANTUM_KEYCODE(code) ((code) >= QK_BOOTLOADER && (code) <= QK_LAYER_LOCK)
 #define IS_KB_KEYCODE(code) ((code) >= QK_KB_0 && (code) <= QK_KB_31)
 #define IS_USER_KEYCODE(code) ((code) >= QK_USER_0 && (code) <= QK_USER_31)
 
@@ -1527,6 +1529,6 @@ enum qk_keycode_defines {
 #define UNDERGLOW_KEYCODE_RANGE             QK_UNDERGLOW_TOGGLE ... QK_UNDERGLOW_SPEED_DOWN
 #define RGB_KEYCODE_RANGE                   RGB_MODE_PLAIN ... RGB_MODE_TWINKLE
 #define RGB_MATRIX_KEYCODE_RANGE            QK_RGB_MATRIX_ON ... QK_RGB_MATRIX_SPEED_DOWN
-#define QUANTUM_KEYCODE_RANGE               QK_BOOTLOADER ... QK_ALT_REPEAT_KEY
+#define QUANTUM_KEYCODE_RANGE               QK_BOOTLOADER ... QK_LAYER_LOCK
 #define KB_KEYCODE_RANGE                    QK_KB_0 ... QK_KB_31
 #define USER_KEYCODE_RANGE                  QK_USER_0 ... QK_USER_31

quantum/layer_lock.c

@@ -0,0 +1,81 @@
+// 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.
+
+#include "layer_lock.h"
+#include "quantum_keycodes.h"
+
+#ifndef NO_ACTION_LAYER
+// The current lock state. The kth bit is on if layer k is locked.
+layer_state_t locked_layers = 0;
+
+// Layer Lock timer to disable layer lock after X seconds inactivity
+#    if defined(LAYER_LOCK_IDLE_TIMEOUT) && LAYER_LOCK_IDLE_TIMEOUT > 0
+uint32_t layer_lock_timer = 0;
+
+void layer_lock_task(void) {
+    if (locked_layers && timer_elapsed32(layer_lock_timer) > LAYER_LOCK_IDLE_TIMEOUT) {
+        layer_lock_all_off();
+        layer_lock_timer = timer_read32();
+    }
+}
+#    endif // LAYER_LOCK_IDLE_TIMEOUT > 0
+
+bool is_layer_locked(uint8_t layer) {
+    return locked_layers & ((layer_state_t)1 << layer);
+}
+
+void layer_lock_invert(uint8_t layer) {
+    const layer_state_t mask = (layer_state_t)1 << layer;
+    if ((locked_layers & mask) == 0) { // Layer is being locked.
+#    ifndef NO_ACTION_ONESHOT
+        if (layer == get_oneshot_layer()) {
+            reset_oneshot_layer(); // Reset so that OSL doesn't turn layer off.
+        }
+#    endif // NO_ACTION_ONESHOT
+        layer_on(layer);
+#    if defined(LAYER_LOCK_IDLE_TIMEOUT) && LAYER_LOCK_IDLE_TIMEOUT > 0
+        layer_lock_timer = timer_read32();
+#    endif   // LAYER_LOCK_IDLE_TIMEOUT > 0
+    } else { // Layer is being unlocked.
+        layer_off(layer);
+    }
+    layer_lock_set_kb(locked_layers ^= mask);
+}
+
+// Implement layer_lock_on/off by deferring to layer_lock_invert.
+void layer_lock_on(uint8_t layer) {
+    if (!is_layer_locked(layer)) {
+        layer_lock_invert(layer);
+    }
+}
+
+void layer_lock_off(uint8_t layer) {
+    if (is_layer_locked(layer)) {
+        layer_lock_invert(layer);
+    }
+}
+
+void layer_lock_all_off(void) {
+    layer_and(~locked_layers);
+    locked_layers = 0;
+    layer_lock_set_kb(locked_layers);
+}
+
+__attribute__((weak)) bool layer_lock_set_kb(layer_state_t locked_layers) {
+    return layer_lock_set_user(locked_layers);
+}
+__attribute__((weak)) bool layer_lock_set_user(layer_state_t locked_layers) {
+    return true;
+}
+#endif // NO_ACTION_LAYER

quantum/layer_lock.h

@@ -0,0 +1,135 @@
+// 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_layer.h"
+#include "action_util.h"
+
+/**
+ * Handler function for Layer Lock.
+ *
+ * In your keymap, define a custom keycode to use for Layer Lock. Then handle
+ * Layer Lock from your `process_record_user` function by calling
+ * `process_layer_lock`, passing your custom keycode for the `lock_keycode` arg:
+ *
+ *     #include "features/layer_lock.h"
+ *
+ *     bool process_record_user(uint16_t keycode, keyrecord_t* record) {
+ *       if (!process_layer_lock(keycode, record, LLOCK)) { return false; }
+ *       // Your macros ...
+ *
+ *       return true;
+ *     }
+ */
+
+#ifndef NO_ACTION_LAYER
+/** Returns true if `layer` is currently locked. */
+bool is_layer_locked(uint8_t layer);
+
+/** Locks and turns on `layer`. */
+void layer_lock_on(uint8_t layer);
+
+/** Unlocks and turns off `layer`. */
+void layer_lock_off(uint8_t layer);
+
+/** Unlocks and turns off all locked layers. */
+void layer_lock_all_off(void);
+
+/** Toggles whether `layer` is locked. */
+void layer_lock_invert(uint8_t layer);
+
+/**
+ * Optional callback that gets called when a layer is locked or unlocked.
+ *
+ * This is useful to represent the current lock state, e.g. by setting an LED or
+ * playing a sound. In your keymap, define
+ *
+ *     void layer_lock_set_user(layer_state_t locked_layers) {
+ *       // Do something like `set_led(is_layer_locked(NAV));`
+ *     }
+ *
+ * @param locked_layers Bitfield in which the kth bit represents whether the
+ *                      kth layer is on.
+ */
+bool layer_lock_set_kb(layer_state_t locked_layers);
+bool layer_lock_set_user(layer_state_t locked_layers);
+
+void layer_lock_task(void);
+#else  // NO_ACTION_LAYER
+static inline bool is_layer_locked(uint8_t layer) {
+    return false;
+}
+static inline void layer_lock_on(uint8_t layer) {}
+static inline void layer_lock_off(uint8_t layer) {}
+static inline void layer_lock_all_off(void) {}
+static inline void layer_lock_invert(uint8_t layer) {}
+static inline bool layer_lock_set_kb(layer_state_t locked_layers) {
+    return true;
+}
+static inline bool layer_lock_set_user(layer_state_t locked_layers) {
+    return true;
+}
+static inline void layer_lock_task(void) {}
+#endif // NO_ACTION_LAYER

quantum/process_keycode/process_layer_lock.c

@@ -0,0 +1,95 @@
+// 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.c
+ * @brief Layer Lock implementation
+ *
+ * For full documentation, see
+ * <https://getreuer.info/posts/keyboards/layer-lock>
+ */
+
+#include "layer_lock.h"
+#include "process_layer_lock.h"
+#include "quantum_keycodes.h"
+#include "action_util.h"
+
+// The current lock state. The kth bit is on if layer k is locked.
+extern layer_state_t locked_layers;
+#if defined(LAYER_LOCK_IDLE_TIMEOUT) && LAYER_LOCK_IDLE_TIMEOUT > 0
+extern uint32_t layer_lock_timer;
+#endif
+
+// Handles an event on an `MO` or `TT` layer switch key.
+static bool handle_mo_or_tt(uint8_t layer, keyrecord_t* record) {
+    if (is_layer_locked(layer)) {
+        if (record->event.pressed) { // On press, unlock the layer.
+            layer_lock_invert(layer);
+        }
+        return false; // Skip default handling.
+    }
+    return true;
+}
+
+bool process_layer_lock(uint16_t keycode, keyrecord_t* record) {
+#ifndef NO_ACTION_LAYER
+#    if defined(LAYER_LOCK_IDLE_TIMEOUT) && LAYER_LOCK_IDLE_TIMEOUT > 0
+    layer_lock_timer = timer_read32();
+#    endif // LAYER_LOCK_IDLE_TIMEOUT > 0
+
+    // The intention is that locked layers remain on. If something outside of
+    // this feature turned any locked layers off, unlock them.
+    if ((locked_layers & ~layer_state) != 0) {
+        layer_lock_set_kb(locked_layers &= layer_state);
+    }
+
+    if (keycode == QK_LAYER_LOCK) {
+        if (record->event.pressed) { // The layer lock key was pressed.
+            layer_lock_invert(get_highest_layer(layer_state));
+        }
+        return false;
+    }
+
+    switch (keycode) {
+        case QK_MOMENTARY ... QK_MOMENTARY_MAX: // `MO(layer)` keys.
+            return handle_mo_or_tt(QK_MOMENTARY_GET_LAYER(keycode), record);
+
+        case QK_LAYER_TAP_TOGGLE ... QK_LAYER_TAP_TOGGLE_MAX: // `TT(layer)`.
+            return handle_mo_or_tt(QK_LAYER_TAP_TOGGLE_GET_LAYER(keycode), record);
+
+        case QK_LAYER_MOD ... QK_LAYER_MOD_MAX: { // `LM(layer, mod)`.
+            uint8_t layer = QK_LAYER_MOD_GET_LAYER(keycode);
+            if (is_layer_locked(layer)) {
+                if (record->event.pressed) { // On press, unlock the layer.
+                    layer_lock_invert(layer);
+                } else { // On release, clear the mods.
+                    clear_mods();
+                    send_keyboard_report();
+                }
+                return false; // Skip default handling.
+            }
+        } break;
+
+#    ifndef NO_ACTION_TAPPING
+        case QK_LAYER_TAP ... QK_LAYER_TAP_MAX: // `LT(layer, key)` keys.
+            if (record->tap.count == 0 && !record->event.pressed && is_layer_locked(QK_LAYER_TAP_GET_LAYER(keycode))) {
+                // Release event on a held layer-tap key where the layer is locked.
+                return false; // Skip default handling so that layer stays on.
+            }
+            break;
+#    endif // NO_ACTION_TAPPING
+    }
+#endif // NO_ACTION_LAYER
+    return true;
+}

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);

quantum/quantum.c

@@ -76,6 +76,10 @@
 #    include "process_unicode_common.h"
 #endif
 
+#ifdef LAYER_LOCK_ENABLE
+#    include "process_layer_lock.h"
+#endif // LAYER_LOCK_ENABLE
+
 #ifdef AUDIO_ENABLE
 #    ifndef GOODBYE_SONG
 #        define GOODBYE_SONG SONG(GOODBYE_SOUND)
@@ -400,6 +404,9 @@ bool process_record_quantum(keyrecord_t *record) {
 #ifdef TRI_LAYER_ENABLE
             process_tri_layer(keycode, record) &&
 #endif
+#ifdef LAYER_LOCK_ENABLE
+            process_layer_lock(keycode, record) &&
+#endif
 #ifdef BLUETOOTH_ENABLE
             process_connection(keycode, record) &&
 #endif

quantum/quantum.h

@@ -240,6 +240,10 @@ extern layer_state_t layer_state;
 #    include "os_detection.h"
 #endif
 
+#ifdef LAYER_LOCK_ENABLE
+#    include "layer_lock.h"
+#endif // LAYER_LOCK_ENABLE
+
 void set_single_default_layer(uint8_t default_layer);
 void set_single_persistent_default_layer(uint8_t default_layer);
 

tests/layer_lock/config.h

@@ -0,0 +1,8 @@
+// Copyright 2021 Christopher Courtney, aka Drashna Jael're  (@drashna) <drashna@live.com>
+// SPDX-License-Identifier: GPL-2.0-or-later
+
+#pragma once
+
+#include "test_common.h"
+
+#define LAYER_LOCK_IDLE_TIMEOUT 1000

tests/layer_lock/test.mk

@@ -0,0 +1,8 @@
+# Copyright 2021 Christopher Courtney, aka Drashna Jael're  (@drashna) <drashna@live.com>
+# SPDX-License-Identifier: GPL-2.0-or-later
+
+# --------------------------------------------------------------------------------
+# Keep this file, even if it is empty, as a marker that this folder contains tests
+# --------------------------------------------------------------------------------
+
+LAYER_LOCK_ENABLE = yes

tests/layer_lock/test_layer_lock.cpp

@@ -0,0 +1,284 @@
+// Copyright 2021 Christopher Courtney, aka Drashna Jael're  (@drashna) <drashna@live.com>
+// SPDX-License-Identifier: GPL-2.0-or-later
+
+#include "keycodes.h"
+#include "test_common.hpp"
+
+using testing::_;
+
+class LayerLock : public TestFixture {};
+
+TEST_F(LayerLock, LayerLockState) {
+    TestDriver driver;
+    KeymapKey  key_a = KeymapKey(0, 0, 0, KC_A);
+    KeymapKey  key_b = KeymapKey(1, 0, 0, KC_B);
+    KeymapKey  key_c = KeymapKey(2, 0, 0, KC_C);
+    KeymapKey  key_d = KeymapKey(3, 0, 0, KC_C);
+
+    set_keymap({key_a, key_b, key_c, key_d});
+
+    EXPECT_FALSE(is_layer_locked(1));
+    EXPECT_FALSE(is_layer_locked(2));
+    EXPECT_FALSE(is_layer_locked(3));
+
+    layer_lock_invert(1); // Layer 1: unlocked -> locked
+    layer_lock_on(2);     // Layer 2: unlocked -> locked
+    layer_lock_off(3);    // Layer 3: stays unlocked
+
+    // Layers 1 and 2 are now on.
+    EXPECT_TRUE(layer_state_is(1));
+    EXPECT_TRUE(layer_state_is(2));
+    // Layers 1 and 2 are now locked.
+    EXPECT_TRUE(is_layer_locked(1));
+    EXPECT_TRUE(is_layer_locked(2));
+    EXPECT_FALSE(is_layer_locked(3));
+
+    layer_lock_invert(1); // Layer 1: locked -> unlocked
+    layer_lock_on(2);     // Layer 2: stays locked
+    layer_lock_on(3);     // Layer 3: unlocked -> locked
+
+    EXPECT_FALSE(layer_state_is(1));
+    EXPECT_TRUE(layer_state_is(2));
+    EXPECT_TRUE(layer_state_is(3));
+    EXPECT_FALSE(is_layer_locked(1));
+    EXPECT_TRUE(is_layer_locked(2));
+    EXPECT_TRUE(is_layer_locked(3));
+
+    layer_lock_invert(1); // Layer 1: unlocked -> locked
+    layer_lock_off(2);    // Layer 2: locked -> unlocked
+
+    EXPECT_TRUE(layer_state_is(1));
+    EXPECT_FALSE(layer_state_is(2));
+    EXPECT_TRUE(layer_state_is(3));
+    EXPECT_TRUE(is_layer_locked(1));
+    EXPECT_FALSE(is_layer_locked(2));
+    EXPECT_TRUE(is_layer_locked(3));
+
+    layer_lock_all_off(); // Layers 1 and 3: locked -> unlocked
+
+    EXPECT_FALSE(layer_state_is(1));
+    EXPECT_FALSE(layer_state_is(2));
+    EXPECT_FALSE(layer_state_is(3));
+    EXPECT_FALSE(is_layer_locked(1));
+    EXPECT_FALSE(is_layer_locked(2));
+    EXPECT_FALSE(is_layer_locked(3));
+}
+
+TEST_F(LayerLock, LayerLockMomentaryTest) {
+    TestDriver driver;
+    KeymapKey  key_layer = KeymapKey(0, 0, 0, MO(1));
+    KeymapKey  key_a     = KeymapKey(0, 1, 0, KC_A);
+    KeymapKey  key_trns  = KeymapKey(1, 0, 0, KC_TRNS);
+    KeymapKey  key_ll    = KeymapKey(1, 1, 0, QK_LAYER_LOCK);
+
+    set_keymap({key_layer, key_a, key_trns, key_ll});
+
+    EXPECT_NO_REPORT(driver);
+    key_layer.press();
+    run_one_scan_loop();
+    EXPECT_TRUE(layer_state_is(1));
+    EXPECT_FALSE(is_layer_locked(1));
+    VERIFY_AND_CLEAR(driver);
+
+    EXPECT_NO_REPORT(driver);
+    tap_key(key_ll);
+    EXPECT_TRUE(layer_state_is(1));
+    EXPECT_TRUE(is_layer_locked(1));
+    VERIFY_AND_CLEAR(driver);
+
+    EXPECT_NO_REPORT(driver);
+    key_layer.release();
+    run_one_scan_loop();
+    EXPECT_TRUE(layer_state_is(1));
+    EXPECT_TRUE(is_layer_locked(1));
+    VERIFY_AND_CLEAR(driver);
+
+    // Pressing Layer Lock again unlocks the lock.
+    EXPECT_NO_REPORT(driver);
+    key_ll.press();
+    run_one_scan_loop();
+    EXPECT_FALSE(layer_state_is(1));
+    EXPECT_FALSE(is_layer_locked(1));
+    VERIFY_AND_CLEAR(driver);
+}
+
+TEST_F(LayerLock, LayerLockLayerTapTest) {
+    TestDriver driver;
+    KeymapKey  key_layer = KeymapKey(0, 0, 0, LT(1, KC_B));
+    KeymapKey  key_a     = KeymapKey(0, 1, 0, KC_A);
+    KeymapKey  key_trns  = KeymapKey(1, 0, 0, KC_TRNS);
+    KeymapKey  key_ll    = KeymapKey(1, 1, 0, QK_LAYER_LOCK);
+
+    set_keymap({key_layer, key_a, key_trns, key_ll});
+
+    EXPECT_NO_REPORT(driver);
+    key_layer.press();
+    idle_for(TAPPING_TERM);
+    run_one_scan_loop();
+    EXPECT_TRUE(layer_state_is(1));
+    VERIFY_AND_CLEAR(driver);
+
+    EXPECT_NO_REPORT(driver);
+    tap_key(key_ll);
+    EXPECT_TRUE(layer_state_is(1));
+    EXPECT_TRUE(is_layer_locked(1));
+    VERIFY_AND_CLEAR(driver);
+
+    // Pressing Layer Lock again unlocks the lock.
+    EXPECT_NO_REPORT(driver);
+    key_ll.press();
+    run_one_scan_loop();
+    EXPECT_FALSE(layer_state_is(1));
+    EXPECT_FALSE(is_layer_locked(1));
+    VERIFY_AND_CLEAR(driver);
+}
+
+TEST_F(LayerLock, LayerLockOneshotTapTest) {
+    TestDriver driver;
+    KeymapKey  key_layer = KeymapKey(0, 0, 0, OSL(1));
+    KeymapKey  key_a     = KeymapKey(0, 1, 0, KC_A);
+    KeymapKey  key_trns  = KeymapKey(1, 0, 0, KC_TRNS);
+    KeymapKey  key_ll    = KeymapKey(1, 1, 0, QK_LAYER_LOCK);
+
+    set_keymap({key_layer, key_a, key_trns, key_ll});
+
+    EXPECT_NO_REPORT(driver);
+    tap_key(key_layer);
+    run_one_scan_loop();
+    EXPECT_TRUE(layer_state_is(1));
+    VERIFY_AND_CLEAR(driver);
+
+    EXPECT_NO_REPORT(driver);
+    tap_key(key_ll);
+    run_one_scan_loop();
+    EXPECT_TRUE(layer_state_is(1));
+    EXPECT_TRUE(is_layer_locked(1));
+    VERIFY_AND_CLEAR(driver);
+
+    // Pressing Layer Lock again unlocks the lock.
+    EXPECT_NO_REPORT(driver);
+    key_ll.press();
+    run_one_scan_loop();
+    EXPECT_FALSE(layer_state_is(1));
+    EXPECT_FALSE(is_layer_locked(1));
+    VERIFY_AND_CLEAR(driver);
+}
+
+TEST_F(LayerLock, LayerLockOneshotHoldTest) {
+    TestDriver driver;
+    KeymapKey  key_layer = KeymapKey(0, 0, 0, OSL(1));
+    KeymapKey  key_a     = KeymapKey(0, 1, 0, KC_A);
+    KeymapKey  key_trns  = KeymapKey(1, 0, 0, KC_TRNS);
+    KeymapKey  key_ll    = KeymapKey(1, 1, 0, QK_LAYER_LOCK);
+
+    set_keymap({key_layer, key_a, key_trns, key_ll});
+
+    EXPECT_NO_REPORT(driver);
+    key_layer.press();
+    idle_for(TAPPING_TERM);
+    run_one_scan_loop();
+    EXPECT_TRUE(layer_state_is(1));
+    VERIFY_AND_CLEAR(driver);
+
+    EXPECT_NO_REPORT(driver);
+    tap_key(key_ll);
+    run_one_scan_loop();
+    EXPECT_TRUE(layer_state_is(1));
+    VERIFY_AND_CLEAR(driver);
+
+    EXPECT_NO_REPORT(driver);
+    key_layer.release();
+    run_one_scan_loop();
+    EXPECT_TRUE(layer_state_is(1));
+    EXPECT_TRUE(is_layer_locked(1));
+    VERIFY_AND_CLEAR(driver);
+
+    // Pressing Layer Lock again unlocks the lock.
+    EXPECT_NO_REPORT(driver);
+    key_ll.press();
+    run_one_scan_loop();
+    EXPECT_FALSE(layer_state_is(1));
+    EXPECT_FALSE(is_layer_locked(1));
+    VERIFY_AND_CLEAR(driver);
+}
+
+TEST_F(LayerLock, LayerLockTimeoutTest) {
+    TestDriver driver;
+    KeymapKey  key_layer = KeymapKey(0, 0, 0, MO(1));
+    KeymapKey  key_a     = KeymapKey(0, 1, 0, KC_A);
+    KeymapKey  key_trns  = KeymapKey(1, 0, 0, KC_TRNS);
+    KeymapKey  key_ll    = KeymapKey(1, 1, 0, QK_LAYER_LOCK);
+
+    set_keymap({key_layer, key_a, key_trns, key_ll});
+
+    EXPECT_NO_REPORT(driver);
+    key_layer.press();
+    run_one_scan_loop();
+    EXPECT_TRUE(layer_state_is(1));
+    VERIFY_AND_CLEAR(driver);
+
+    EXPECT_NO_REPORT(driver);
+    tap_key(key_ll);
+    EXPECT_TRUE(layer_state_is(1));
+    VERIFY_AND_CLEAR(driver);
+
+    EXPECT_NO_REPORT(driver);
+    key_layer.release();
+    run_one_scan_loop();
+    EXPECT_TRUE(layer_state_is(1));
+    EXPECT_TRUE(is_layer_locked(1));
+    VERIFY_AND_CLEAR(driver);
+
+    EXPECT_NO_REPORT(driver);
+    idle_for(LAYER_LOCK_IDLE_TIMEOUT);
+    run_one_scan_loop();
+    EXPECT_FALSE(layer_state_is(1));
+    EXPECT_FALSE(is_layer_locked(1));
+    VERIFY_AND_CLEAR(driver);
+}
+
+TEST_F(LayerLock, ToKeyOverridesLayerLock) {
+    TestDriver driver;
+    KeymapKey  key_layer = KeymapKey(0, 0, 0, MO(1));
+    KeymapKey  key_to0   = KeymapKey(1, 0, 0, TO(0));
+    KeymapKey  key_ll    = KeymapKey(1, 1, 0, QK_LAYER_LOCK);
+
+    set_keymap({key_layer, key_to0, key_ll});
+
+    EXPECT_NO_REPORT(driver);
+    layer_lock_on(1);
+    run_one_scan_loop();
+    EXPECT_TRUE(layer_state_is(1));
+    EXPECT_TRUE(is_layer_locked(1));
+    VERIFY_AND_CLEAR(driver);
+
+    EXPECT_NO_REPORT(driver);
+    tap_key(key_to0); // TO(0) overrides Layer Lock and unlocks layer 1.
+    EXPECT_FALSE(layer_state_is(1));
+    EXPECT_FALSE(is_layer_locked(1));
+    VERIFY_AND_CLEAR(driver);
+}
+
+TEST_F(LayerLock, LayerClearOverridesLayerLock) {
+    TestDriver driver;
+    KeymapKey  key_layer = KeymapKey(0, 0, 0, MO(1));
+    KeymapKey  key_a     = KeymapKey(0, 1, 0, KC_A);
+    KeymapKey  key_ll    = KeymapKey(1, 1, 0, QK_LAYER_LOCK);
+
+    set_keymap({key_layer, key_a, key_ll});
+
+    EXPECT_NO_REPORT(driver);
+    layer_lock_on(1);
+    run_one_scan_loop();
+    EXPECT_TRUE(layer_state_is(1));
+    EXPECT_TRUE(is_layer_locked(1));
+    VERIFY_AND_CLEAR(driver);
+
+    EXPECT_REPORT(driver, (KC_A));
+    layer_clear(); // layer_clear() overrides Layer Lock and unlocks layer 1.
+    key_a.press();
+    run_one_scan_loop();
+    EXPECT_FALSE(layer_state_is(1));
+    EXPECT_FALSE(is_layer_locked(1));
+    VERIFY_AND_CLEAR(driver);
+}

tests/test_common/keycode_table.cpp

@@ -699,6 +699,7 @@ std::map<uint16_t, std::string> KEYCODE_ID_TABLE = {
     {QK_TRI_LAYER_UPPER, "QK_TRI_LAYER_UPPER"},
     {QK_REPEAT_KEY, "QK_REPEAT_KEY"},
     {QK_ALT_REPEAT_KEY, "QK_ALT_REPEAT_KEY"},
+    {QK_LAYER_LOCK, "QK_LAYER_LOCK"},
     {QK_KB_0, "QK_KB_0"},
     {QK_KB_1, "QK_KB_1"},
     {QK_KB_2, "QK_KB_2"},