Have you noticed a small new button in the bottom left corner of IntelliJ IDEA window since version 10.5?
It lets you have more space in the main frame and give more prominence to the editor by hiding the tool window bars.
With the tool buttons hidden you, of course, still can access the tool windows using their numeric hotkeys, e.g. Alt+1 for Project view, Alt+9 for Changes, etc.
But if you need to refresh them in your mind, or want to open one with the mouse, you can show the tool window buttons just for a moment. To do that, hit the Alt key twice (Command key on Mac), and keep it down — while the key is pressed, the tool window buttons are visible. Release the key — and they disappear again, leaving your main frame large and spacious.