I don't think minimizable is supposed to do anything out of the box.
From the docks ( 3 and 4 ) for Window,
True to display the 'minimize' tool button and allow the user to minimize the window, false to hide the button and disallow minimizing the window (defaults to false). Note that this button provides no implementation -- the behavior of minimizing a window is implementation-specific, so the minimize event must be handled and a custom minimize behavior implemented for this option to be useful.
That's interesting. Didn't know it worked like that. I soon realized that the option I was looking for is "collapsible". After much thought... I see why they did that... it's for "Porlet" min/max view.