Does it also happen when all other applications (including browsers) are closed?
If not, you may open the other applications one by one until you find the intruder.
Still the same problem. Closed all programs and still have this issue. Also rebooted, still same issue...
Previously another user with this issue discovered it was due to some thrid party mouse application.. right zoom or click or something.
I tried using the Guest Account on my Mac. In the Guest Account, Illustrator works perfect!
Then I started wondering what processes (tools/tweaks) I run by default on my account. Turned out the problem was called by a little background tool called RightZoom. RightZoom let's the green 'zoom' button always maximize your current window.
So thanks! Problem solved!
Haha what a coincidence. Figurred it out myself at the same time...