[Core] Tri Layer Keys (#19795)

Co-authored-by: wilba <wilba@wilba.tech>
Co-authored-by: Pablo Martínez <58857054+elpekenin@users.noreply.github.com>
Co-authored-by: Joel Challis <git@zvecr.com>
Co-authored-by: Nick Brassel <nick@tzarc.org>

19 files changed, 377 insertions, 15 deletions

diff --git a/builddefs/common_features.mk b/builddefs/common_features.mk
index 4e032f9dcf..0880934e95 100644
--- a/builddefs/common_features.mk
+++ b/builddefs/common_features.mk
@@ -620,6 +620,7 @@ ifeq ($(strip $(VIA_ENABLE)), yes)
     DYNAMIC_KEYMAP_ENABLE := yes
     RAW_ENABLE := yes
     BOOTMAGIC_ENABLE := yes
+    TRI_LAYER_ENABLE := yes
     SRC += $(QUANTUM_DIR)/via.c
     OPT_DEFS += -DVIA_ENABLE
 endif
diff --git a/builddefs/generic_features.mk b/builddefs/generic_features.mk
index 0d897bc1c8..5a1ef5c6f0 100644
--- a/builddefs/generic_features.mk
+++ b/builddefs/generic_features.mk
@@ -39,6 +39,7 @@ GENERIC_FEATURES = \
     VELOCIKEY \
     WPM \
     DYNAMIC_TAPPING_TERM \
+    TRI_LAYER
 
 define HANDLE_GENERIC_FEATURE
     # $$(info "Processing: $1_ENABLE $2.c")
diff --git a/builddefs/show_options.mk b/builddefs/show_options.mk
index e9f7e7d047..9723b45438 100644
--- a/builddefs/show_options.mk
+++ b/builddefs/show_options.mk
@@ -84,7 +84,8 @@ OTHER_OPTION_NAMES = \
   PROGRAMMABLE_BUTTON_ENABLE \
   SECURE_ENABLE \
   CAPS_WORD_ENABLE \
-  AUTOCORRECT_ENABLE
+  AUTOCORRECT_ENABLE \
+  TRI_LAYER_ENABLE
 
 define NAME_ECHO
        @printf "  %-30s = %-16s # %s\\n" "$1" "$($1)" "$(origin $1)"
diff --git a/data/constants/keycodes/keycodes_0.0.2_quantum.hjson b/data/constants/keycodes/keycodes_0.0.2_quantum.hjson
new file mode 100644
index 0000000000..960afa4e51
--- /dev/null
+++ b/data/constants/keycodes/keycodes_0.0.2_quantum.hjson
@@ -0,0 +1,18 @@
+{
+    "keycodes": {
+        "0x7C77": {
+            "group": "quantum",
+            "key": "QK_TRI_LAYER_LOWER",
+            "aliases": [
+                "TL_LOWR"
+            ]
+        },
+        "0x7C78": {
+            "group": "quantum",
+            "key": "QK_TRI_LAYER_UPPER",
+            "aliases": [
+                "TL_UPPR"
+            ]
+        }
+    }
+}
diff --git a/docs/_summary.md b/docs/_summary.md
index d2d4bb9b32..b038f08a4d 100644
--- a/docs/_summary.md
+++ b/docs/_summary.md
@@ -93,6 +93,7 @@
     * [Swap Hands](feature_swap_hands.md)
     * [Tap Dance](feature_tap_dance.md)
     * [Tap-Hold Configuration](tap_hold.md)
+    * [Tri Layer](feature_tri_layer.md)
     * [Unicode](feature_unicode.md)
     * [Userspace](feature_userspace.md)
     * [WPM Calculation](feature_wpm.md)
diff --git a/docs/feature_tri_layer.md b/docs/feature_tri_layer.md
new file mode 100644
index 0000000000..aa6c87719c
--- /dev/null
+++ b/docs/feature_tri_layer.md
@@ -0,0 +1,48 @@
+# Tri Layers :id=tri-layers
+
+This enables support for the OLKB style "Tri Layer" keycodes.  These function similar to the `MO` (momentary) function key, but if both the "Lower" and "Upper" keys are pressed, it activates a third "Adjust" layer.  To enable this functionality, add this line to your `rules.mk`:
+
+```make
+TRI_LAYER_ENABLE = yes
+```
+
+Note that the "upper", "lower" and "adjust" names don't have a particular significance, they are just used to identify and clarify the behavior. Layers are processed from highest numeric value to lowest, however the values are not required to be consecutive.
+
+For a detailed explanation of how the layer stack works, check out [Keymap Overview](keymap.md#keymap-and-layers).
+
+## Keycodes :id=keycodes
+
+| Keycode              | Alias     | Description                                                                                             |
+|----------------------|-----------|---------------------------------------------------------------------------------------------------------|
+| `QK_TRI_LAYER_LOWER` | `TL_LOWR` | Momentarily enables the "lower" layer. Enables the "adjust" layer if the "upper" layer is also enabled" |
+| `QK_TRI_LAYER_UPPER` | `TL_UPPR` | Momentarily enables the "upper" layer. Enables the "adjust" layer if the "lower" layer is also enabled" |
+
+## Configuration
+
+To change the default values for the layers, you can change these defines, in your `config.h`
+
+| Config name              | Default | Description                              |
+|--------------------------|---------|------------------------------------------|
+| `TRI_LAYER_LOWER_LAYER`  | `1`     | Sets the default for the "lower" layer.  |
+| `TRI_LAYER_UPPER_LAYER`  | `2`     | Sets the default for the "upper" layer.  |
+| `TRI_LAYER_ADJUST_LAYER` | `3`     | Sets the default for the "adjust" layer. |
+
+Eg, if you wanted to set the "Adjust" layer to be layer 5, you'd add this to your `config.h`: 
+
+```c
+#define TRI_LAYER_ADJUST_LAYER 5
+```
+
+## Functions
+
+| Function name                                |  Description                                    |
+|----------------------------------------------|-------------------------------------------------|
+| `set_tri_layer_lower_layer(layer)`           | Changes the "lower" layer*.                     |
+| `set_tri_layer_upper_layer(layer)`           | Changes the "upper" layer*.                     |
+| `set_tri_layer_adjust_layer(layer)`          | Changes the "adjust" layer*.                    |
+| `set_tri_layer_layers(lower, upper, adjust)` | Stes the "lower", "upper" and "adjust" layers*. |
+| `get_tri_layer_lower_layer()`                | Gets the current "lower" layer.                 |
+| `get_tri_layer_upper_layer()`                | Gets the current "upper" layer.                 |
+| `get_tri_layer_adjust_layer()`               | Gets the current "adjust" layer.                |
+
+!> Note: these settings are not persisent, and will be reset to the default on power loss or power cycling of the controller.
diff --git a/quantum/action_layer.c b/quantum/action_layer.c
index 74e8137920..35f494df8f 100644
--- a/quantum/action_layer.c
+++ b/quantum/action_layer.c
@@ -349,3 +349,13 @@ uint8_t layer_switch_get_layer(keypos_t key) {
 action_t layer_switch_get_action(keypos_t key) {
     return action_for_key(layer_switch_get_layer(key), key);
 }
+
+layer_state_t update_tri_layer_state(layer_state_t state, uint8_t layer1, uint8_t layer2, uint8_t layer3) {
+    layer_state_t mask12 = ((layer_state_t)1 << layer1) | ((layer_state_t)1 << layer2);
+    layer_state_t mask3  = (layer_state_t)1 << layer3;
+    return (state & mask12) == mask12 ? (state | mask3) : (state & ~mask3);
+}
+
+void update_tri_layer(uint8_t layer1, uint8_t layer2, uint8_t layer3) {
+    layer_state_set(update_tri_layer_state(layer_state, layer1, layer2, layer3));
+}
diff --git a/quantum/action_layer.h b/quantum/action_layer.h
index 3fe2726529..ff783bb3e7 100644
--- a/quantum/action_layer.h
+++ b/quantum/action_layer.h
@@ -113,6 +113,25 @@ void          layer_and(layer_state_t state);
 void          layer_xor(layer_state_t state);
 layer_state_t layer_state_set_user(layer_state_t state);
 layer_state_t layer_state_set_kb(layer_state_t state);
+
+/**
+ * @brief Applies the tri layer to global layer state. Not be used in layer_state_set_(kb|user) functions.
+ *
+ * @param layer1 First layer to check for tri layer
+ * @param layer2 Second layer to check for tri layer
+ * @param layer3 Layer to activate if both other layers are enabled
+ */
+void update_tri_layer(uint8_t layer1, uint8_t layer2, uint8_t layer3);
+/**
+ * @brief Applies the tri layer behavior to supplied layer bitmask, without using layer functions.
+ *
+ * @param state Original layer bitmask to check and modify
+ * @param layer1 First layer to check for tri layer
+ * @param layer2 Second layer to check for tri layer
+ * @param layer3 Layer to activate if both other layers are enabled
+ * @return layer_state_t returns a modified layer bitmask with tri layer modifications applied
+ */
+layer_state_t update_tri_layer_state(layer_state_t state, uint8_t layer1, uint8_t layer2, uint8_t layer3);
 #else
 #    define layer_state 0
 
@@ -131,6 +150,8 @@ layer_state_t layer_state_set_kb(layer_state_t state);
 #    define layer_xor(state) (void)state
 #    define layer_state_set_kb(state) (void)state
 #    define layer_state_set_user(state) (void)state
+#    define update_tri_layer(layer1, layer2, layer3)
+#    define update_tri_layer_state(state, layer1, layer2, layer3) (void)state
 #endif
 
 /* pressed actions cache */
diff --git a/quantum/keycodes.h b/quantum/keycodes.h
index f24ccf01b8..4fa4306ed6 100644
--- a/quantum/keycodes.h
+++ b/quantum/keycodes.h
@@ -717,6 +717,8 @@ enum qk_keycode_defines {
     QK_AUTOCORRECT_ON = 0x7C74,
     QK_AUTOCORRECT_OFF = 0x7C75,
     QK_AUTOCORRECT_TOGGLE = 0x7C76,
+    QK_TRI_LAYER_LOWER = 0x7C77,
+    QK_TRI_LAYER_UPPER = 0x7C78,
     SAFE_RANGE = 0x7E00,
 
 // Alias
@@ -1282,6 +1284,8 @@ enum qk_keycode_defines {
     AC_ON      = QK_AUTOCORRECT_ON,
     AC_OFF     = QK_AUTOCORRECT_OFF,
     AC_TOGG    = QK_AUTOCORRECT_TOGGLE,
+    TL_LOWR    = QK_TRI_LAYER_LOWER,
+    TL_UPPR    = QK_TRI_LAYER_UPPER,
 };
 
 // Range Helpers
@@ -1333,4 +1337,4 @@ enum qk_keycode_defines {
 #define IS_MACRO_KEYCODE(code) ((code) >= QK_MACRO_0 && (code) <= QK_MACRO_31)
 #define IS_BACKLIGHT_KEYCODE(code) ((code) >= QK_BACKLIGHT_ON && (code) <= QK_BACKLIGHT_TOGGLE_BREATHING)
 #define IS_RGB_KEYCODE(code) ((code) >= RGB_TOG && (code) <= RGB_MODE_TWINKLE)
-#define IS_QUANTUM_KEYCODE(code) ((code) >= QK_BOOTLOADER && (code) <= QK_AUTOCORRECT_TOGGLE)
+#define IS_QUANTUM_KEYCODE(code) ((code) >= QK_BOOTLOADER && (code) <= QK_TRI_LAYER_UPPER)
diff --git a/quantum/process_keycode/process_tri_layer.c b/quantum/process_keycode/process_tri_layer.c
new file mode 100644
index 0000000000..1e681b9a1c
--- /dev/null
+++ b/quantum/process_keycode/process_tri_layer.c
@@ -0,0 +1,30 @@
+// Copyright 2023 QMK
+// SPDX-License-Identifier: GPL-2.0-or-later
+
+#include "process_tri_layer.h"
+#include "tri_layer.h"
+#include "action_layer.h"
+
+bool process_tri_layer(uint16_t keycode, keyrecord_t *record) {
+    switch (keycode) {
+        case QK_TRI_LAYER_LOWER:
+            if (record->event.pressed) {
+                layer_on(get_tri_layer_lower_layer());
+                update_tri_layer(get_tri_layer_lower_layer(), get_tri_layer_upper_layer(), get_tri_layer_adjust_layer());
+            } else {
+                layer_off(get_tri_layer_lower_layer());
+                update_tri_layer(get_tri_layer_lower_layer(), get_tri_layer_upper_layer(), get_tri_layer_adjust_layer());
+            }
+            return false;
+        case QK_TRI_LAYER_UPPER:
+            if (record->event.pressed) {
+                layer_on(get_tri_layer_upper_layer());
+                update_tri_layer(get_tri_layer_lower_layer(), get_tri_layer_upper_layer(), get_tri_layer_adjust_layer());
+            } else {
+                layer_off(get_tri_layer_upper_layer());
+                update_tri_layer(get_tri_layer_lower_layer(), get_tri_layer_upper_layer(), get_tri_layer_adjust_layer());
+            }
+            return false;
+    }
+    return true;
+}
diff --git a/quantum/process_keycode/process_tri_layer.h b/quantum/process_keycode/process_tri_layer.h
new file mode 100644
index 0000000000..9c4e3df1c2
--- /dev/null
+++ b/quantum/process_keycode/process_tri_layer.h
@@ -0,0 +1,16 @@
+// Copyright 2023 QMK
+// SPDX-License-Identifier: GPL-2.0-or-later
+
+#pragma once
+
+#include "action.h"
+
+/**
+ * @brief Handles tri layer behavior
+ *
+ * @param keycode the keycode
+ * @param record the key record structure
+ * @return true continue handling keycodes
+ * @return false stop handling keycodes
+ */
+bool process_tri_layer(uint16_t keycode, keyrecord_t *record);
diff --git a/quantum/quantum.c b/quantum/quantum.c
index 5554ecab32..0587f215fe 100644
--- a/quantum/quantum.c
+++ b/quantum/quantum.c
@@ -343,6 +343,9 @@ bool process_record_quantum(keyrecord_t *record) {
 #ifdef AUTOCORRECT_ENABLE
             process_autocorrect(keycode, record) &&
 #endif
+#ifdef TRI_LAYER_ENABLE
+            process_tri_layer(keycode, record) &&
+#endif
             true)) {
         return false;
     }
@@ -443,16 +446,6 @@ void set_single_persistent_default_layer(uint8_t default_layer) {
     default_layer_set((layer_state_t)1 << default_layer);
 }
 
-layer_state_t update_tri_layer_state(layer_state_t state, uint8_t layer1, uint8_t layer2, uint8_t layer3) {
-    layer_state_t mask12 = ((layer_state_t)1 << layer1) | ((layer_state_t)1 << layer2);
-    layer_state_t mask3  = (layer_state_t)1 << layer3;
-    return (state & mask12) == mask12 ? (state | mask3) : (state & ~mask3);
-}
-
-void update_tri_layer(uint8_t layer1, uint8_t layer2, uint8_t layer3) {
-    layer_state_set(update_tri_layer_state(layer_state, layer1, layer2, layer3));
-}
-
 //------------------------------------------------------------------------------
 // Override these functions in your keymap file to play different tunes on
 // different events such as startup and bootloader jump
diff --git a/quantum/quantum.h b/quantum/quantum.h
index 615ec2382c..708d325a32 100644
--- a/quantum/quantum.h
+++ b/quantum/quantum.h
@@ -240,9 +240,10 @@ extern layer_state_t layer_state;
 #    include "process_autocorrect.h"
 #endif
 
-// For tri-layer
-void          update_tri_layer(uint8_t layer1, uint8_t layer2, uint8_t layer3);
-layer_state_t update_tri_layer_state(layer_state_t state, uint8_t layer1, uint8_t layer2, uint8_t layer3);
+#ifdef TRI_LAYER_ENABLE
+#    include "tri_layer.h"
+#    include "process_tri_layer.h"
+#endif
 
 void set_single_persistent_default_layer(uint8_t default_layer);
 
diff --git a/quantum/tri_layer.c b/quantum/tri_layer.c
new file mode 100644
index 0000000000..a5e3f8cb47
--- /dev/null
+++ b/quantum/tri_layer.c
@@ -0,0 +1,39 @@
+// Copyright 2023 QMK
+// SPDX-License-Identifier: GPL-2.0-or-later
+
+#include "tri_layer.h"
+#include <stdint.h>
+
+static uint8_t tri_layer_lower_layer  = TRI_LAYER_LOWER_LAYER;
+static uint8_t tri_layer_upper_layer  = TRI_LAYER_UPPER_LAYER;
+static uint8_t tri_layer_adjust_layer = TRI_LAYER_ADJUST_LAYER;
+
+void set_tri_layer_lower_layer(uint8_t layer) {
+    tri_layer_lower_layer = layer;
+}
+
+void set_tri_layer_upper_layer(uint8_t layer) {
+    tri_layer_upper_layer = layer;
+}
+
+void set_tri_layer_adjust_layer(uint8_t layer) {
+    tri_layer_adjust_layer = layer;
+}
+
+void set_tri_layer_layers(uint8_t lower, uint8_t raise, uint8_t adjust) {
+    tri_layer_lower_layer  = lower;
+    tri_layer_upper_layer  = raise;
+    tri_layer_adjust_layer = adjust;
+}
+
+uint8_t get_tri_layer_lower_layer(void) {
+    return tri_layer_lower_layer;
+}
+
+uint8_t get_tri_layer_upper_layer(void) {
+    return tri_layer_upper_layer;
+}
+
+uint8_t get_tri_layer_adjust_layer(void) {
+    return tri_layer_adjust_layer;
+}
diff --git a/quantum/tri_layer.h b/quantum/tri_layer.h
new file mode 100644
index 0000000000..3341ebffb2
--- /dev/null
+++ b/quantum/tri_layer.h
@@ -0,0 +1,59 @@
+// Copyright 2023 QMK
+// SPDX-License-Identifier: GPL-2.0-or-later
+
+#include <stdint.h>
+
+#ifndef TRI_LAYER_LOWER_LAYER
+#    define TRI_LAYER_LOWER_LAYER 1
+#endif
+#ifndef TRI_LAYER_UPPER_LAYER
+#    define TRI_LAYER_UPPER_LAYER 2
+#endif
+#ifndef TRI_LAYER_ADJUST_LAYER
+#    define TRI_LAYER_ADJUST_LAYER 3
+#endif
+
+/**
+ * @brief Set the tri layer lower layer index
+ *
+ * @param layer
+ */
+void set_tri_layer_lower_layer(uint8_t layer);
+/**
+ * @brief Set the tri layer upper layer index
+ *
+ * @param layer
+ */
+void set_tri_layer_upper_layer(uint8_t layer);
+/**
+ * @brief Set the tri layer adjust layer index
+ *
+ * @param layer
+ */
+void set_tri_layer_adjust_layer(uint8_t layer);
+/**
+ * @brief Set the tri layer indices
+ *
+ * @param lower
+ * @param upper
+ * @param adjust
+ */
+void set_tri_layer_layers(uint8_t lower, uint8_t upper, uint8_t adjust);
+/**
+ * @brief Get the tri layer lower layer index
+ *
+ * @return uint8_t
+ */
+uint8_t get_tri_layer_lower_layer(void);
+/**
+ * @brief Get the tri layer upper layer index
+ *
+ * @return uint8_t
+ */
+uint8_t get_tri_layer_upper_layer(void);
+/**
+ * @brief Get the tri layer adjust layer index
+ *
+ * @return uint8_t
+ */
+uint8_t get_tri_layer_adjust_layer(void);
diff --git a/tests/test_common/keycode_table.cpp b/tests/test_common/keycode_table.cpp
index ab9c292e95..337bc8fb5f 100644
--- a/tests/test_common/keycode_table.cpp
+++ b/tests/test_common/keycode_table.cpp
@@ -659,5 +659,7 @@ std::map<uint16_t, std::string> KEYCODE_ID_TABLE = {
     {QK_AUTOCORRECT_ON, "QK_AUTOCORRECT_ON"},
     {QK_AUTOCORRECT_OFF, "QK_AUTOCORRECT_OFF"},
     {QK_AUTOCORRECT_TOGGLE, "QK_AUTOCORRECT_TOGGLE"},
+    {QK_TRI_LAYER_LOWER, "QK_TRI_LAYER_LOWER"},
+    {QK_TRI_LAYER_UPPER, "QK_TRI_LAYER_UPPER"},
     {SAFE_RANGE, "SAFE_RANGE"},
 };
diff --git a/tests/tri_layer/config.h b/tests/tri_layer/config.h
new file mode 100644
index 0000000000..b68bf0c2d5
--- /dev/null
+++ b/tests/tri_layer/config.h
@@ -0,0 +1,6 @@
+// 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"
diff --git a/tests/tri_layer/test.mk b/tests/tri_layer/test.mk
new file mode 100644
index 0000000000..50548c3e1c
--- /dev/null
+++ b/tests/tri_layer/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
+# --------------------------------------------------------------------------------
+
+TRI_LAYER_ENABLE = yes
diff --git a/tests/tri_layer/test_tri_layer.cpp b/tests/tri_layer/test_tri_layer.cpp
new file mode 100644
index 0000000000..fffc124f4c
--- /dev/null
+++ b/tests/tri_layer/test_tri_layer.cpp
@@ -0,0 +1,103 @@
+// Copyright 2021 Christopher Courtney, aka Drashna Jael're  (@drashna) <drashna@live.com>
+// SPDX-License-Identifier: GPL-2.0-or-later
+
+#include "test_common.hpp"
+
+using testing::_;
+using testing::InSequence;
+
+class TriLayer : public TestFixture {};
+
+TEST_F(TriLayer, TriLayerLowerTest) {
+    TestDriver driver;
+    KeymapKey  lower_layer_key = KeymapKey{0, 0, 0, QK_TRI_LAYER_LOWER};
+
+    set_keymap({lower_layer_key, KeymapKey{1, 0, 0, KC_TRNS}});
+
+    /* Press Lower. */
+    EXPECT_NO_REPORT(driver);
+    lower_layer_key.press();
+    run_one_scan_loop();
+    EXPECT_TRUE(layer_state_is(get_tri_layer_lower_layer()));
+    EXPECT_FALSE(layer_state_is(get_tri_layer_upper_layer()));
+    EXPECT_FALSE(layer_state_is(get_tri_layer_adjust_layer()));
+    VERIFY_AND_CLEAR(driver);
+
+    /* Release Lower. */
+    EXPECT_NO_REPORT(driver);
+    lower_layer_key.release();
+    run_one_scan_loop();
+    EXPECT_FALSE(layer_state_is(get_tri_layer_lower_layer()));
+    EXPECT_FALSE(layer_state_is(get_tri_layer_upper_layer()));
+    EXPECT_FALSE(layer_state_is(get_tri_layer_adjust_layer()));
+    VERIFY_AND_CLEAR(driver);
+}
+
+TEST_F(TriLayer, TriLayerUpperTest) {
+    TestDriver driver;
+    KeymapKey  upper_layer_key = KeymapKey{0, 0, 0, QK_TRI_LAYER_UPPER};
+
+    set_keymap({upper_layer_key, KeymapKey{2, 0, 0, KC_TRNS}});
+
+    /* Press Raise. */
+    EXPECT_NO_REPORT(driver);
+    upper_layer_key.press();
+    run_one_scan_loop();
+    EXPECT_FALSE(layer_state_is(get_tri_layer_lower_layer()));
+    EXPECT_TRUE(layer_state_is(get_tri_layer_upper_layer()));
+    EXPECT_FALSE(layer_state_is(get_tri_layer_adjust_layer()));
+    VERIFY_AND_CLEAR(driver);
+
+    /* Release Raise. */
+    EXPECT_NO_REPORT(driver);
+    upper_layer_key.release();
+    run_one_scan_loop();
+    EXPECT_FALSE(layer_state_is(get_tri_layer_lower_layer()));
+    EXPECT_FALSE(layer_state_is(get_tri_layer_upper_layer()));
+    EXPECT_FALSE(layer_state_is(get_tri_layer_adjust_layer()));
+    VERIFY_AND_CLEAR(driver);
+}
+
+TEST_F(TriLayer, TriLayerAdjustTest) {
+    TestDriver driver;
+    KeymapKey  lower_layer_key = KeymapKey{0, 0, 0, QK_TRI_LAYER_LOWER};
+    KeymapKey  upper_layer_key = KeymapKey{0, 1, 0, QK_TRI_LAYER_UPPER};
+
+    set_keymap({
+        upper_layer_key,
+        lower_layer_key,
+        KeymapKey{1, 0, 0, KC_TRNS},
+        KeymapKey{1, 1, 0, KC_TRNS},
+        KeymapKey{2, 0, 0, KC_TRNS},
+        KeymapKey{2, 1, 0, KC_TRNS},
+        KeymapKey{3, 0, 0, KC_TRNS},
+        KeymapKey{3, 1, 0, KC_TRNS},
+    });
+
+    /* Press Lower, then upper, and release upper and then lower. */
+    EXPECT_NO_REPORT(driver);
+    lower_layer_key.press();
+    run_one_scan_loop();
+    EXPECT_TRUE(layer_state_is(get_tri_layer_lower_layer()));
+    EXPECT_FALSE(layer_state_is(get_tri_layer_upper_layer()));
+    EXPECT_FALSE(layer_state_is(get_tri_layer_adjust_layer()));
+
+    upper_layer_key.press();
+    run_one_scan_loop();
+    EXPECT_TRUE(layer_state_is(get_tri_layer_lower_layer()));
+    EXPECT_TRUE(layer_state_is(get_tri_layer_upper_layer()));
+    EXPECT_TRUE(layer_state_is(get_tri_layer_adjust_layer()));
+
+    lower_layer_key.release();
+    run_one_scan_loop();
+    EXPECT_FALSE(layer_state_is(get_tri_layer_lower_layer()));
+    EXPECT_TRUE(layer_state_is(get_tri_layer_upper_layer()));
+    EXPECT_FALSE(layer_state_is(get_tri_layer_adjust_layer()));
+
+    upper_layer_key.release();
+    run_one_scan_loop();
+    EXPECT_FALSE(layer_state_is(get_tri_layer_lower_layer()));
+    EXPECT_FALSE(layer_state_is(get_tri_layer_upper_layer()));
+    EXPECT_FALSE(layer_state_is(get_tri_layer_adjust_layer()));
+    VERIFY_AND_CLEAR(driver);
+}