Finder Take Control From Other Programs

If your issue is the same as mine (and it took me since El Capitan to figure it out), it isn't the Finder. It was poorly written apps that check in the background for updates to third party apps I had installed. All for the sole purpose of being able to gleefully display the message "There's a new update available!" the next time I opened that app.

Check these three locations.

The main Library folder (the one next to Applications and System at the root of the drive):

/Library/LaunchAgents

/Library/LaunchDaemons

The Library folder in your user account:

/Users/name_of_account/Library/LaunchAgents

What you're looking for is a .plist file that (hopefully) gives you an indication in the name that it's for updates. I removed the ones for Wacom, X-Rite and Corel (Toast Titanium) and restarted.

No more mysterious jumps away from the foreground app.

Focus-stealing phenomenon such as you describe can occur if a previously loaded nonwindowed process becomes active. According to Apple's programming guidelines it's considered impolite to do that, at best.

It's up to you to determine what it is though. Activity Monitor can be used to identify active as well as inactive processes, but if you have no idea and don't know where to begin, consider using EtreCheck. It gathers and presents a variety of Information that may help identify the cause. At your option post its report in a reply, using the "Additional Text" icon:

Thanks so much, this was super helpful... I went in to those folders and deleted some old app references, and some other things you mentioned as well, and this seems to have solved my problem. This has been frustrating me for quite some time. I hunted down and ran a python script to help me sort the focus stealer out, only to find that it was "Finder". I assumed it must be some third part app thing, but didn't know where to go from there, so luckily I tripped over this thread.

I used that script for a while, too. It never helped find anything because the non-windowed app would jump in and out so quickly, the next time the script checked, it would only report what had come back to the foreground, and not what had pulled the focus away from the app you were using.

It's seems like the Finder, but as John mentioned, it's a very poorly written app (like these update checkers) that aren't supposed to come to the foreground at all. But they do, and having no application window to display causes the OS to show something else as the foreground app.