Position Panel Above Text moved a floating Tabs panel only in the layout's
in-window rect. On desktop builds a floating panel is an OS window, which
moves only when its desktop rect changes, so the panel stayed where it
was and the layout followed it back on the next frame. A docked panel
floated above the text as expected.
place_floating now also sets the window's desktop rect, from the main
window's content origin, when the panel's window already has one.