[Core] Flow Tap tap-hold option to disable HRMs during fast typing (#25125)

aka Global Quick Tap, Require Prior Idle

quantum/action_tapping.h

@@ -111,6 +111,63 @@ char chordal_hold_handedness(keypos_t key);
 extern const char chordal_hold_layout[MATRIX_ROWS][MATRIX_COLS] PROGMEM;
 #endif
 
+#ifdef FLOW_TAP_TERM
+/**
+ * Callback to specify the keys where Flow Tap is enabled.
+ *
+ * Flow Tap is constrained to certain keys by the following rule: this callback
+ * is called for both the tap-hold key *and* the key press immediately preceding
+ * it. If the callback returns true for both keycodes, Flow Tap is enabled.
+ *
+ * The default implementation of this callback corresponds to
+ *
+ *     bool is_flow_tap_key(uint16_t keycode) {
+ *       switch (get_tap_keycode(keycode)) {
+ *         case KC_SPC:
+ *         case KC_A ... KC_Z:
+ *         case KC_DOT:
+ *         case KC_COMM:
+ *         case KC_SCLN:
+ *         case KC_SLSH:
+ *           return true;
+ *       }
+ *       return false;
+ *     }
+ *
+ * @param keycode Keycode of the key.
+ * @return Whether to enable Flow Tap for this key.
+ */
+bool is_flow_tap_key(uint16_t keycode);
+
+/**
+ * Callback to customize Flow Tap filtering.
+ *
+ * Flow Tap acts only when key events are closer together than this time.
+ *
+ * Return a time of 0 to disable filtering. In this way, Flow Tap may be
+ * disabled for certain tap-hold keys, or when following certain previous keys.
+ *
+ * The default implementation of this callback is
+ *
+ *     uint16_t get_flow_tap_term(uint16_t keycode, keyrecord_t* record,
+ *                                uint16_t prev_keycode) {
+ *       if (is_flow_tap_key(keycode) && is_flow_tap_key(prev_keycode)) {
+ *         return g_flow_tap_term;
+ *       }
+ *       return 0;
+ *     }
+ *
+ * NOTE: If both `is_flow_tap_key()` and `get_flow_tap_term()` are defined, then
+ * `get_flow_tap_term()` takes precedence.
+ *
+ * @param keycode Keycode of the tap-hold key.
+ * @param record keyrecord_t of the tap-hold event.
+ * @param prev_keycode Keycode of the previously pressed key.
+ * @return Time in milliseconds.
+ */
+uint16_t get_flow_tap_term(uint16_t keycode, keyrecord_t *record, uint16_t prev_keycode);
+#endif // FLOW_TAP_TERM
+
 #ifdef DYNAMIC_TAPPING_TERM_ENABLE
 extern uint16_t g_tapping_term;
 #endif