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.

#include QMK_KEYBOARD_H bool lctl_other_key_pressed = false; uint16_t lctl_hold_timer = 0; bool process_record_user(uint16_t keycode, keyrecord_t *record) { switch (keycode) { // Left control. Can be changed to any modifier, or MO({LAYER_NUM}) for layer-taps. case KC_LCTL: if (record->event.pressed) { // Records press time. lctl_hold_timer = timer_read(); // At the beginning, no other key is pressed. lctl_other_key_pressed = false; } else if (timer_elapsed(lctl_timer) < 500 && !space_other_key_pressed) { // Sends out 'A' if the key is held for less than 0.5s and no other key was pressed during the period. tap_code(KC_A); } break; default: // Another key is pressed. lctl_other_key_pressed = true; } return true; };

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.