if you are referring to the musicbee application itself you can already collapse the left panel by clicking the button at the bottom left of the navigator panel
otherwise i dont understand what you mean
Yeap, I know about that button, but I think it would be nice, if you add another button with same functionality.
But this button have some difference in appearance:
*it's located in the left edge of the window;
*width = 4-5 px. /height = (size of window) - (sizes of title/menubar/playing controls/ etc.). Just like in that related image.
think now you'll understand me.