President Donald Trump has repeatedly tested the boundaries of executive power since his return to office and is exploring uncharted legal territory with threats to carry out mass firings during the federal government shutdown, legal experts and lawmakers said. The Republican president this year has aggressively pursued cuts to the federal government and has repeatedly raised the prospect of using the shutdown, now in its seventh day, as justification for permanent layoffs. But his administration has not acted on it. That, experts said, may be because neither federal courts nor the federal employee board that oversees workers has determined whether federal agencies may permanently cut staff during a shutdown. "We are in largely uncharted territory," Nick Bednar, a University of...