Display Underscan ineffective after reboot

I can use Display Settings, Underscan, to make the screen fit my display properly, but after rebooting, the under scan setting isn’t working—reverting to no underscan. Yet Display Settings, Underscan does show how I set it before rebooting. It just isn’t effective after reboot. Stranger still, moving the cursor to the Underscan field causes the display to correct itself, without actually changing the setting. That is, merely clicking the setting causes it to take effect.