I knew that any opened file tab can be dragged and opened as an undocked window, but I was never able to put the window back in the panel's top line. Today, accidentally I succeeded, now I want to tell for those who do not know:
move the mouse icon over the name of the file, NOT over the horizontal "bar" anchored at the top of the window with the WeBulder sign, just below it!
Now in the middle of the window appears the crossed windows image with the possible positions, just put in the center window!