Fix layer switching from tap dances by redoing the keymap lookup

quantum/process_keycode/process_tap_dance.c

@@ -115,12 +115,12 @@ static inline void process_tap_dance_action_on_dance_finished(qk_tap_dance_actio
     }
 }
 
-void preprocess_tap_dance(uint16_t keycode, keyrecord_t *record) {
+bool preprocess_tap_dance(uint16_t keycode, keyrecord_t *record) {
     qk_tap_dance_action_t *action;
 
-    if (!record->event.pressed) return;
+    if (!record->event.pressed) return false;
 
-    if (!active_td || keycode == active_td) return;
+    if (!active_td || keycode == active_td) return false;
 
     action                             = &tap_dance_actions[TD_INDEX(active_td)];
     action->state.interrupted          = true;
@@ -130,6 +130,12 @@ void preprocess_tap_dance(uint16_t keycode, keyrecord_t *record) {
     // Tap dance actions can leave some weak mods active (e.g., if the tap dance is mapped to a keycode with
     // modifiers), but these weak mods should not affect the keypress which interrupted the tap dance.
     clear_weak_mods();
+
+    // Signal that a tap dance has been finished due to being interrupted,
+    // therefore the keymap lookup for the currently processed event needs to
+    // be repeated with the current layer state that might have been updated by
+    // the finished tap dance.
+    return true;
 }
 
 bool process_tap_dance(uint16_t keycode, keyrecord_t *record) {

quantum/process_keycode/process_tap_dance.h

@@ -81,7 +81,7 @@ void reset_tap_dance(qk_tap_dance_state_t *state);
 
 /* To be used internally */
 
-void preprocess_tap_dance(uint16_t keycode, keyrecord_t *record);
+bool preprocess_tap_dance(uint16_t keycode, keyrecord_t *record);
 bool process_tap_dance(uint16_t keycode, keyrecord_t *record);
 void tap_dance_task(void);
 

quantum/quantum.c

@@ -251,7 +251,11 @@ bool process_record_quantum(keyrecord_t *record) {
 #endif
 
 #ifdef TAP_DANCE_ENABLE
-    preprocess_tap_dance(keycode, record);
+    if (preprocess_tap_dance(keycode, record)) {
+        // The tap dance might have updated the layer state, therefore the
+        // result of the keycode lookup might change.
+        keycode = get_record_keycode(record, true);
+    }
 #endif
 
     if (!(

tests/tap_dance/tap_dance_layers/config.h

@@ -0,0 +1,6 @@
+// Copyright 2022 Sergey Vlasov (@sigprof)
+// SPDX-License-Identifier: GPL-2.0-or-later
+
+#pragma once
+
+#include "test_common.h"

tests/tap_dance/tap_dance_layers/tap_dance_defs.c

@@ -0,0 +1,97 @@
+// Copyright 2022 Sergey Vlasov (@sigprof)
+// SPDX-License-Identifier: GPL-2.0-or-later
+
+#include "quantum.h"
+#include "tap_dance_defs.h"
+
+// Implement custom keycodes which are used to check that the layer switching
+// behaves properly.
+bool process_record_user(uint16_t keycode, keyrecord_t *record) {
+    switch (keycode) {
+        case FAST_AB:
+        case SLOW_AB:
+            if (record->event.pressed) {
+                tap_code(KC_A);
+            } else {
+                tap_code(KC_B);
+            }
+            return keycode == SLOW_AB;
+        case FAST_CD:
+        case SLOW_CD:
+            if (record->event.pressed) {
+                tap_code(KC_C);
+            } else {
+                tap_code(KC_D);
+            }
+            return keycode == SLOW_CD;
+    }
+    return true;
+}
+
+// Implement a custom tap dance with the following behavior:
+// - single tap: KC_APP
+// - single hold: MO(1)
+// - double tap/hold: KC_RCTL
+// (The single tap and hold actions are mostly equivalent to LT(1, KC_APP).)
+
+enum lt_app_state {
+    LTA_NONE,
+    LTA_SINGLE_TAP,
+    LTA_SINGLE_HOLD,
+    LTA_DOUBLE_HOLD,
+};
+
+static enum lt_app_state saved_lt_app_state;
+
+static enum lt_app_state get_lt_app_state(qk_tap_dance_state_t *state) {
+    if (state->count == 1) {
+        if (!state->pressed) {
+            return LTA_SINGLE_TAP;
+        } else {
+            return LTA_SINGLE_HOLD;
+        }
+    } else if (state->count == 2) {
+        return LTA_DOUBLE_HOLD;
+    } else {
+        return LTA_NONE;
+    }
+}
+
+static void lt_app_finished(qk_tap_dance_state_t *state, void *user_data) {
+    saved_lt_app_state = get_lt_app_state(state);
+    switch (saved_lt_app_state) {
+        case LTA_NONE:
+            break;
+        case LTA_SINGLE_TAP:
+            register_code(KC_APP);
+            break;
+        case LTA_SINGLE_HOLD:
+            layer_on(1);
+            break;
+        case LTA_DOUBLE_HOLD:
+            register_code(KC_RCTL);
+            break;
+    }
+}
+
+static void lt_app_reset(qk_tap_dance_state_t *state, void *user_data) {
+    switch (saved_lt_app_state) {
+        case LTA_NONE:
+            break;
+        case LTA_SINGLE_TAP:
+            unregister_code(KC_APP);
+            break;
+        case LTA_SINGLE_HOLD:
+            layer_off(1);
+            break;
+        case LTA_DOUBLE_HOLD:
+            unregister_code(KC_RCTL);
+            break;
+    }
+}
+
+qk_tap_dance_action_t tap_dance_actions[] = {
+    [TD_L_MOVE] = ACTION_TAP_DANCE_LAYER_MOVE(KC_APP, 1),
+    [TD_L_TOGG] = ACTION_TAP_DANCE_LAYER_TOGGLE(KC_APP, 1),
+    [TD_LT_APP] = ACTION_TAP_DANCE_FN_ADVANCED(NULL, lt_app_finished, lt_app_reset),
+};

tests/tap_dance/tap_dance_layers/tap_dance_defs.h

@@ -0,0 +1,29 @@
+// Copyright 2022 Sergey Vlasov (@sigprof)
+// SPDX-License-Identifier: GPL-2.0-or-later
+
+#pragma once
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+enum custom_keycodes {
+    // (FAST|SLOW)_xy = tap KC_x on press, tap KC_y on release.  For FAST_xy
+    // process_record_user() returns false to stop processing early; for
+    // SLOW_xy process_record_user() returns true, therefore all other key
+    // handlers are invoked.
+    FAST_AB = SAFE_RANGE,
+    FAST_CD,
+    SLOW_AB,
+    SLOW_CD,
+};
+
+enum tap_dance_ids {
+    TD_L_MOVE, // ACTION_TAP_DANCE_LAYER_MOVE(KC_APP, 1)
+    TD_L_TOGG, // ACTION_TAP_DANCE_LAYER_TOGGLE(KC_APP, 1)
+    TD_LT_APP, // similar to LT(1, KC_APP) with KC_RCTL on tap+hold or double tap
+};
+
+#ifdef __cplusplus
+}
+#endif

tests/tap_dance/tap_dance_layers/test.mk

@@ -0,0 +1,10 @@
+# Copyright 2022 Sergey Vlasov (@sigprof)
+# SPDX-License-Identifier: GPL-2.0-or-later
+
+# --------------------------------------------------------------------------------
+# Keep this file, even if it is empty, as a marker that this folder contains tests
+# --------------------------------------------------------------------------------
+
+TAP_DANCE_ENABLE = yes
+
+SRC += tap_dance_defs.c