Sorry, this did not work either. I clicked and dragged all around the border. Still full screen. I -CAN- use Alt-Tab to change to other windows, read mail and surf while Spotify is playing in the backgroud. But I can do these things correctly in windowed mode by using Treacle and Pandora.
Linux Mint 17.3, this does not work. Spotify is in full screen and refuses to switch to windowed mode.
I could not work around this issue with Alt-F11 either in Unity or Gnome on my Ubuntu installation. In Unity on Ubuntu you can hold alt and grab the window with the mouse and drag it make it windowed again. In Gnome this does not work for me.
I treat this as a bug in the Spotify client and it is tracked in our bug tracker and will be fixed eventually.
The only workaround that I know that always work is removing the window prefs and restarting the client.
I got a situation were my Spotify window was invisible due to a size of 1x1 (width and height). The music was playing and I did not know how to stop that. I noticed the window title was changing with each song using: wmctrl -l
So I could do something like: wmctrl -r Sting -e 0,45,62,800,800
Restoring to a window 800x800 nearly top left on my screen. Obviously a song by Sting was playing.