macOS 10.13.6, Safari 12.1, running on a MBP (Retina) mid-2014. The system software and Safari are both up-to-date and the App Store shows no updates available.
Safari partially loads, however, when user "MM" logs in: without any user input, the Safari icon bounces in the Dock and the black dot appears below the icon. The Safari window does not appear, and the word "Safari" does not appear in the Menu Bar. If I double-click on the Dock icon, Safari loads completely, its window appears, and it seems to run correctly.
Safari does appear in the "Force Quit" menu (Apple Menu > Force Quit); when running either partially or fully, if Force Quit is selected, the dot disappears...and then the icon bounces, and the dot reappears. Activity Monitor shows a Safari process running; if forced-quit from within Activity Monitor, the dot disappears, then the icon bounces, the dot reappears, and the process reappears in Activity Monitor.
I created a Test User account, and the spontaneous partial launch does not occur when Test User logs in. It looks like there's something awry with user MM's files.
I have reinstalled macOS using Recovery. The odd behavior does not occur for user MM when the computer is started in Safe Mode. Manually deleting Safari caches from both the root level Library and the user level Library for account MM had no effect. Turning off the two Safari extensions had no effect. There are no automatic login items in user account MM.
I'm stumped. How can I further diagnose this, and what might I do to solve it?