Photos app stuck on Repairing Library… (104% complete)

I have a late 2014 Mac Mini running OS 10.14.6 with a 2 TB SSD drive. Yesterday morning when I launched Photos, it told me it needed to update/repair the photo library. It seemed to be fine, but as of the evening it appeared to be stuck at Repairing Library… (104% complete). Now the next day, it's still stuck on the same message.

Activity Monitor doesn't show the Photos app as 'Not Responding'.

I don't believe anything has changed recently prior to this, although it is possible that Software Update may have applied a Security Update recently and rebooted the computer since the last time I had run Photos.

Is there any way that I can verify that the app is still 'working'? Do I dare Force Quit?