Compile and start the dockwidgetts example (Src/qtbase/examples/widgets/mainwindows/dockwidgets).
Click on the "Maximize" button: Make the QMainWindow fullscreen size.
Click the "Paragraphs" QDockWidget and move it into the upper dockwidget area: Make it full monitor width and docked.
Then click the same QDockWidget again and move it out of it's dockwidget area in order to get it free floating.

Result:
The QDockWidget is free floating.
Its width is the monitor width and cannot be changed.
It is not possible to move it in X direction even if the desktop is extended to a second monitor. It is not possible to move it to the second monitor then.
Trying to make it smaller reasults in drawing errors.