How to create a float window floating above all other windows just like twitter?

The twitter app has a float window when you click t开发者_如何学Gohe contextual menu "Tweet",is it a NSPanel? How to?

Take a look at NSWindow's setLevel: method:

Each level in the list groups windows within it in front of those in all preceding groups. Floating windows, for example, appear in front of all normal-level windows. When a window enters a new level, it’s ordered in front of all its peers in that level.

-- from NSWindow's setLevel: reference.

You may also find Matt Gemmell's MAAttachedWindow project helpful:

This can be achieved with an NSWindow subclass. Check Matt Gemmell's custom MAAttachedWindow class here.