I traced a little bit, seems when the control is at floating state, it is on a control which is called FloatingWindowContain. And it is inheriance from Forms. And the FormBorderStyle is a toolWindow. Thus, by default the floating window does not have maxmize/minimize button.

In my appliciation, I want the default behavier to be the floating window will have the max/min button. 

Here is what I did:

at runtime, when a dockable panel is changing to a floating state, I get the floating window control (which is FloatingWindowContain) using FindForm() method and modify the FormBorderStyle to Sizable. The floating window will show the max/min button.  

 However, is there any cleaner ways to do it?