I think this is discussed already here in the forum. And there are plans to improve docker in future.
One thing you are assuming wrong is equating the window tab to that for the docker. Both are different in Qt the toolkit that krita uses to show the UI. And it is because of this toolkit there are limitations for tweaking it like adding close button next to the title etc.
And there is some other library is in consideration which provides better functionality for the docker. it might take long to develop so I would advice some patience.
You can search for past discussion