+ * Parameters:
+ * ENTER_DELAY |=======|
+ * SEND_FN_TERM |================|
+ *
+ * Fn key processing cases:
+ * 1. release Fn after SEND_FN_TERM.
+ * Layer sw ___________|~~~~~~~~~~~|___
+ * Fn press ___|~~~~~~~~~~~~~~~~~~~|___
+ * Fn send ___________________________
+ *
+ * 2. release Fn during SEND_FN_TERM.(not layer used)
+ * Layer sw ___________|~~~~~~|________
+ * Fn press ___|~~~~~~~~~~~~~~|________
+ * Fn key send __________________|~|______
+ * other key press ___________________________
+ * other key send ___________________________
+ *
+ * 3. release Fn during SEND_FN_TERM.(layer used)
+ * Layer sw ___________|~~~~~~|________
+ * Fn press ___|~~~~~~~~~~~~~~|________
+ * Fn key send ___________________________
+ * Fn send ___________________________
+ * other key press _____________|~~|__________
+ * other key send _____________|~~|__________
+ *
+ * 4. press other key during ENTER_DELAY.
+ * Layer sw ___________________________
+ * Fn key press ___|~~~~~~~~~|_____________
+ * Fn key send ______|~~~~~~|_____________
+ * other key press ______|~~~|________________
+ * other key send _______|~~|________________
+ *
+ * 5. press Fn while press other key.
+ * Layer sw ___________________________
+ * Fn key press ___|~~~~~~~~~|_____________
+ * Fn key send ___|~~~~~~~~~|_____________
+ * other key press ~~~~~~~|___________________
+ * other key send ~~~~~~~|___________________
+ *
+ * 6. press Fn twice quickly and keep holding down.(repeat)
+ * Layer sw ___________________________
+ * Fn key press ___|~|____|~~~~~~~~~~~~~~~~
+ * Fn key send _____|~|__|~~~~~~~~~~~~~~~~