[Feature] Send a dummy keycode to neutralize flashing modifiers in retro tap and key overrides (#20992)

quantum/action.c

@@ -528,6 +528,13 @@ void process_action(keyrecord_t *record, action_t action) {
                             unregister_code(action.key.code);
                         } else {
                             ac_dprintf("MODS_TAP: No tap: add_mods\n");
+#    if defined(RETRO_TAPPING) && defined(DUMMY_MOD_NEUTRALIZER_KEYCODE)
+                            // Send a dummy keycode to neutralize flashing modifiers
+                            // if the key was held and then released with no interruptions.
+                            if (retro_tapping_counter == 2) {
+                                neutralize_flashing_modifiers(get_mods());
+                            }
+#    endif
                             unregister_mods(mods);
                         }
                     }

quantum/action_util.c

@@ -500,3 +500,28 @@ __attribute__((weak)) void oneshot_layer_changed_kb(uint8_t layer) {
 uint8_t has_anymod(void) {
     return bitpop(real_mods);
 }
+
+#ifdef DUMMY_MOD_NEUTRALIZER_KEYCODE
+/** \brief Send a dummy keycode in between the register and unregister event of a modifier key, to neutralize the "flashing modifiers" phenomenon.
+ *
+ * \param active_mods 8-bit packed bit-array describing the currently active modifiers (in the format GASCGASC).
+ *
+ * Certain QMK features like  key overrides or retro tap must unregister a previously
+ * registered modifier before sending another keycode but this can trigger undesired
+ * keyboard shortcuts if the clean tap of a single modifier key is bound to an action
+ * on the host OS, as is for example the case for the left GUI key on Windows, which
+ * opens the Start Menu when tapped.
+ */
+void neutralize_flashing_modifiers(uint8_t active_mods) {
+    // In most scenarios, the flashing modifiers phenomenon is a problem
+    // only for a subset of modifier masks.
+    const static uint8_t mods_to_neutralize[] = MODS_TO_NEUTRALIZE;
+    const static uint8_t n_mods               = ARRAY_SIZE(mods_to_neutralize);
+    for (uint8_t i = 0; i < n_mods; ++i) {
+        if (active_mods == mods_to_neutralize[i]) {
+            tap_code(DUMMY_MOD_NEUTRALIZER_KEYCODE);
+            break;
+        }
+    }
+}
+#endif

quantum/action_util.h

@@ -102,6 +102,19 @@ void use_oneshot_swaphands(void);
 void clear_oneshot_swaphands(void);
 #endif
 
+#ifdef DUMMY_MOD_NEUTRALIZER_KEYCODE
+// KC_A is used as the lowerbound instead of QK_BASIC because the range QK_BASIC...KC_A includes
+// internal keycodes like KC_NO and KC_TRANSPARENT which are unsuitable for use with `tap_code(kc)`.
+#    if !(KC_A <= DUMMY_MOD_NEUTRALIZER_KEYCODE && DUMMY_MOD_NEUTRALIZER_KEYCODE <= QK_BASIC_MAX)
+#        error "DUMMY_MOD_NEUTRALIZER_KEYCODE must be a basic, unmodified, HID keycode!"
+#    endif
+void neutralize_flashing_modifiers(uint8_t active_mods);
+#endif
+#ifndef MODS_TO_NEUTRALIZE
+#    define MODS_TO_NEUTRALIZE \
+        { MOD_BIT(KC_LEFT_ALT), MOD_BIT(KC_LEFT_GUI) }
+#endif
+
 #ifdef __cplusplus
 }
 #endif

quantum/process_keycode/process_key_override.c

@@ -322,6 +322,15 @@ static bool try_activating_override(const uint16_t keycode, const uint8_t layer,
 
         clear_active_override(false);
 
+#ifdef DUMMY_MOD_NEUTRALIZER_KEYCODE
+        // Send a dummy keycode before unregistering the modifier(s)
+        // so that suppressing the modifier(s) doesn't falsely get interpreted
+        // by the host OS as a tap of a modifier key.
+        // For example, unintended activations of the start menu on Windows when
+        // using a GUI+<kc> key override with suppressed mods.
+        neutralize_flashing_modifiers(active_mods);
+#endif
+
         active_override                 = override;
         active_override_trigger_is_down = true;