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Most of the time an undocked window is just an OSX window with no title but with an alternative "title bar" underneath. Furthermore, that "title bar" has the close button *in the opposite* way compared to the OSX close button. Naturally I would expect in my screenshot (see attachment) that the "native" window title isn't empty text but "Tasks" and that fake "title bar" underneath goes away altogether.
Created attachment 88631 [details] Example of undocked window on OSX
Removing the title bar could be problematic, since dragging the titlebar is what signals to NetBeans that the user might be trying to re-dock the window back into the main window. I could imagine an implementation that didn't have the titlebar and listened on the window for changes in its location, but the results could be confusing, as the user might just be dragging the window and happen to overlap the main window and be unpleasantly surprised if NetBeans assumes that means they want to re-dock it. Might be worth prototyping, though. It definitely would make sense not to show a close button if there is only one tab in the window, and the window has its own close button - that shouldn't be terribly hard.