Taiwan's stock index tumbled the most on record, putting it on track for a bear market as trading resumed after a holiday.The benchmark Taiex fell as much as 9.8% on Monday, taking its decline from a July peak to over 20%.Taiwan Semiconductor Manufacturing Co. (TSMC), which has the top weighting in the index, slid by the exchange's daily limit of 10%.Concerns about geopolitical tensions and artificial-intelligence investments have weighed on Taiwan's stocks this year.