Structure sidebar tab not staying at the top of the page

I have used PHPStorm for a long time and only recently did I start having an issue I can't solve. I"m used to seeing the tabs for Project and below it Structure, at the top left side of the page (not the actual pane, the tab to open the pane).

Now, every time I open the application, it shows Project at top, and at the bottom left Structure (and below that Favorites). I can drag the Structure tab up to the top and it stays there, but every time I close PHPStorm and reopen its back at the bottom again.