Keymap: Hold for MO and Tap for TG (qmk#3742)

Special thanks to Drashna for teaching me how to implement myFNX command. This allows a user to hold Fn for regular Fn and
tap to toggle another layer.

For now this will stay within my community layout as this is the
only layout that I will be using this in. This may change in the future.

layouts/community/60_ansi/mechmerlin-ansi/keymap.c

@@ -4,20 +4,24 @@
 #define FN     1
 #define ARROWS 2
 
+enum keycodes {
+    KC_FNX, // Hold for FN layer, tap to toggle ARROWS layer.
+};
+
 const uint16_t PROGMEM keymaps[][MATRIX_ROWS][MATRIX_COLS] = {
 [BASE] = LAYOUT_60_ansi(
     KC_GESC, KC_1,    KC_2, KC_3, KC_4, KC_5, KC_6, KC_7,  KC_8,    KC_9,    KC_0,    KC_MINS,  KC_EQL,  KC_BSPC, \
 	KC_TAB,  KC_Q,    KC_W, KC_E, KC_R, KC_T, KC_Y, KC_U,  KC_I,    KC_O,    KC_P,    KC_LBRC,  KC_RBRC, KC_BSLASH, \
     KC_LCTL, KC_A,    KC_S, KC_D, KC_F, KC_G, KC_H, KC_J,  KC_K,    KC_L,    KC_SCLN, KC_QUOT,  KC_ENT,  \
     KC_LSFT, KC_Z,    KC_X, KC_C, KC_V, KC_B, KC_N, KC_M,  KC_COMM, KC_DOT,  KC_SLSH, KC_RSFT, \
-    KC_LCTL, KC_LGUI, KC_LALT,       KC_SPACE,             MO(1),   KC_RALT, KC_RGUI, KC_RCTL),
+    KC_LCTL, KC_LGUI, KC_LALT,       KC_SPACE,             KC_FNX,   KC_RALT, KC_RGUI, KC_RCTL),
 
 [FN] = LAYOUT_60_ansi(
     KC_GRV,  KC_F1,   KC_F2,   KC_F3,   KC_F4,   KC_F5,   KC_F6,   KC_F7,   KC_F8,   KC_F9,   KC_F10,  KC_F11,  KC_F12,  KC_DEL, \
 	RGB_TOG, KC_TRNS, KC_TRNS, KC_TRNS, RESET,   KC_TRNS, KC_TRNS, KC_TRNS, KC_TRNS, KC_TRNS, KC_TRNS, KC_TRNS, KC_TRNS, KC_TRNS,  \
     KC_CAPS, KC_TRNS, KC_TRNS, KC_TRNS, KC_TRNS, KC_TRNS, KC_TRNS, KC_TRNS, KC_TRNS, KC_TRNS, KC_TRNS, KC_TRNS, KC_TRNS,  \
     KC_TRNS, KC_TRNS, KC_TRNS, KC_TRNS, KC_TRNS, KC_TRNS, KC_TRNS, KC_MUTE, KC_VOLD, KC_VOLU, KC_TRNS, KC_TRNS, \
-    KC_TRNS, KC_TRNS, KC_TRNS,       TG(2),                                 KC_TRNS, KC_TRNS, KC_TRNS, KC_TRNS),
+    KC_TRNS, KC_TRNS, KC_TRNS,       KC_TRNS,                                 KC_TRNS, KC_TRNS, KC_TRNS, KC_TRNS),
 
 [ARROWS] = LAYOUT_60_ansi(
     KC_TRNS, KC_TRNS, KC_TRNS, KC_TRNS, KC_TRNS, KC_TRNS, KC_TRNS, KC_TRNS, KC_TRNS, KC_TRNS, KC_TRNS, KC_TRNS, KC_TRNS, KC_TRNS, \
@@ -27,3 +31,22 @@ const uint16_t PROGMEM keymaps[][MATRIX_ROWS][MATRIX_COLS] = {
     KC_TRNS, KC_TRNS, KC_TRNS,       KC_TRNS,                               KC_TRNS, KC_LEFT, KC_DOWN, KC_RIGHT)
 
  };
+
+bool process_record_user(uint16_t keycode, keyrecord_t *record) {
+    static uint16_t fnx_layer_timer;
+
+    switch (keycode){
+        case KC_FNX:
+            if(record->event.pressed){
+                fnx_layer_timer = timer_read();
+                layer_on(FN);
+            } else {
+                layer_off(FN);
+                if (timer_elapsed(fnx_layer_timer) < 150) {
+                    layer_invert(ARROWS);
+                }
+            }
+        return false;
+    }
+    return true;
+}