The problem started with not being able to customize the sidebar. No matter what I did to it, it kept reverting to what it had been before I made any change every time I restarted the computer. I had read to delete the file com.apple.sidebarlists.plist in ~/Library/Preferences and then restart to fix it, because it would write a new clean copy of the file. I did. It didn't fix it. In fact, after I restarted, the system didn't write a new com.apple.sidebarlists.plist at all, so I had none at all.
While going through a lot of different things trying to track down the problem I noticed that my "Recent Items" list under the Apple menu wasn't updating.
I happened to discover that the last time the preferences file com.apple.recentitems.plist had been changed was the same date as my deleted com.apple.sidebarlists.plist had been changed. I then discovered that all the following preferences files had a "Date Modified" that was the same as those two:
I have no idea what is going on. I've done everything I know to determine that it isn't a permissions issue on the Preferences folder. I'm the owner, and I have Read and Write access. But the Finder still will not save the sidebar settings. I tried restoring an old copy of com.apple.sidebarlists.plist and com.apple.recentitems.plist from Time Machine, but the same thing happens: I customize the sidebar, log out and back in (or reboot), and it's as though I never customized it at all. It's back the way it was. The "Date Modified" date doesn't change.
I found one, and only one, other discussion describing a similar issue, but it was closed back in 2010, so I can't post to it:
It describes a complex fix that I can't follow and make sense of. I don't know how to set up a new user and "migrate" over the settings from an old user. I have so many preference set for so many things that are crucial to what I do, and I cannot risk losing them, but that discussion certainly seems to indicate that is exactly what can happen, or it is something that would take days. (Reboot each time after copying over every single preferences file? Seriously? You can't be serious.)
Does anybody have any clue what can be causing this pernicious behavior, or any idea what can be done about it that wouldn't be as Draconian as the advice in that thread above?
I've also opened a bunch of different things since I restored an old com.apple.recentitems.plist, and the "Date Modified" doesn't change on it, either—but for some reason, the list under the Apple menu is changing. I haven't tried rebooting yet since I restored the old one, though, and I have a feeling that as soon as I reboot, it will be showing the list from the restored date.
Exhausted and dismayed...