QMK ModTap Alt+Tab

/*  https://www.reddit.com/r/olkb/comments/qfkzan/quick_qmk_question/
 *  Code to include a macro key that is normally ALT, but sends ALT+TAB on tap.
 *  This goes into keymap.c
 *
 *  If you press another key while ALT_TAB is held down, this code assumes you're using ALT like normal
 *  Otherwise, it will tap ALT+TAB if you've held ALT_TAB for less than TAPPING_TERM.
 *  This defaults to 200ms, and can be changed by defining it in config.h with   #define TAPPING_TERM 150
 */

enum custom_keycodes {
    ALT_TAB = SAFE_RANGE,   //the custom macro key that you'll place into your keymap
    //other macros,
};


static uint16_t ALT_TAB_timer = 0;  //declare a timer variable to track hold time for ALT_TAB
static bool key_pressed = false;    //variable used to check if a key was pressed while ALT_TAB was held

bool process_record_user(uint16_t keycode, keyrecord_t *record) {
    if (record->event.pressed)
        key_pressed = true;  //if any key is pressed, set key_pressed to true

    switch(keycode) {
        case ALT_TAB:
            if (record->event.pressed) {        //when ALT_TAB is pressed
                register_code(KC_LALT);         //  press left alt
                ALT_TAB_timer = timer_read();   //  mark the time ALT_TAB is pressed
                key_pressed = false;            //  set key_pressed to false
            } else {                            //when ALT_TAB is released
                unregister_code(KC_LALT);       //  release left alt

                if (key_pressed)                //if another key was pressed while ALT_TAB was held,
                    return false;               //  do nothing else

                if (timer_elapsed(ALT_TAB_timer) < TAPPING_TERM) {  //if ALT_TAB was held for less than TAPPING_TERM
                    tap_code16( LALT(KC_TAB) );                     //  tap ALT+TAB
                }
            }
            break;
    }

    return true;
}