Can't resize IntelliJ window I'm running OSX Yosemite and using JetBrains IntelliJ 2016.2.3.

I can't resize the IDE window, except for the maximize or minimize buttons. What I want to do is use the mouse to stretch the borders of the window to make it smaller or larger. But as I hover my mouse over the borders or corners of the IntelliJ window, the cursor does not change and I can't grab the edge of the window at all.

Every other application window on this Macbook supports resizing normally, but somehow IntelliJ doesn't. 

Strangely, I also run on the same Macbook JetBrains PhpStorm—another IDE product from the same company—and that window does support window-edge resizing.

I've read about some add-on products for OSX that assist in window resizing, but I'd prefer not to install a tool. This should work with plain OSX, shouldn't it?
I did eventually fix this, but it's still a mystery to me why it happened.

I shut down the IDE app completely, and made sure I am updated to the latest patch, and restarted. Now the window resize controls work!

I'm still interested in learning how it got into the state where the resize controls were not selectable, or if possible, how I can fix that without restarting the app.