Admittedly, this is probably the least important bug in the history of software. :-)
Recently, while performance tuning my system, I had occasion to stop/restart IDEA many times in succession. After the last restart, I noticed that the divider between the Structure panel and the editor panel was significantly further to the right than I had originally positioned it. I thought maybe I just didn't remember repositioning it. However, I tried again, paying particular attention to the divider position, and noticed that the divider does indeed move to the right on each restart.
I attached a couple of screen captures with a pixel ruler displayed for reference. Clip1 shows the Structure panel at 200 pixels wide just before I closed IDEA. Clip2 shows the structure panel at 204 pixels wide after simply exiting IDEA and restarting.
I'm running Maia 10781.