The code doesn’ work as expected. Please refer to IGNORE_MOD_TAP_INTERRUPT and PERMISSIVE_HOLD here for possible workarounds.

QMK is the most powerful and popular keyboard firmware in custimzed mechanical keyboard community. It allows you to can write C code to set macros to any key, which makes it highly playable.

Apart from programming, QMK has quite a lot of pre-set quantum keys. An important category of them is mod-tap keys. That means, when you tap the key, it prints a normal output, such as a letter, digit or any other key on the keyboard; when you hold it, it works like a mod key, such as Control, Alt, Shift or Win (Command/Super). That’s particually useful for mini keyboards (40% layout, for example), where keys are highly reused.

However, the performance of mod-tap may not meet the need of fast typing. For example, if the mod-tap key is Control(A), and you press Control(A) and C fast, it actually prints “AC” on the screen, but not sends a Control + C combination. The same thing happens on layer-tap key.

I haven’t read the source code of mod-tap keys, but with programming, we can avoid this issue completely.

 1#include QMK_KEYBOARD_H
 2
 3bool lctl_other_key_pressed = false;
 4uint16_t lctl_hold_timer = 0;
 5
 6bool process_record_user(uint16_t keycode, keyrecord_t *record) {
 7  switch (keycode) {
 8    // Left control. Can be changed to any modifier, or MO({LAYER_NUM}) for layer-taps.
 9    case KC_LCTL:
10      if (record->event.pressed) {
11        // Records press time.
12		lctl_hold_timer = timer_read();
13        // At the beginning, no other key is pressed.
14		lctl_other_key_pressed = false;
15      } else if (timer_elapsed(lctl_timer) < 500 && !space_other_key_pressed) {
16        // Sends out 'A' if the key is held for less than 0.5s and no other key was pressed during the period. 
17		tap_code(KC_A);
18	  }
19      break;
20	default:
21      // Another key is pressed.
22      lctl_other_key_pressed = true;
23  }
24  return true;
25};

In this piece of code, we set the keycode of the mod-tap key to a normal Control key. It ensures it works perfectly as a mod key. Then we attach another event to this key when it is released - we sends a normal letter, which is the “tap” part of mod-tap. Tap is only triggerred when the key was held shortly and no other key is pressed before release. This way, even fast typists can enjoy mod-tap keys happily.

I’m not sure if the current behavior of mod-tap is expected by QMK developer, but I will do some investigation on it and see if I can make the solution general to the default mod-tap implementation.