To replicate, run qtcreator, then open, build and run the dockwidgets example. Tear away one of the docked widgets, drop it somewhere so it's floating, then pick it up and move it. If the mouse is moved quickly the cursor will jump away, and the widget will stop moving. Release the mouse button and move the cursor back over the widget's title bar, and the widget will start following the cursor again, until you click and release with the cursor on the title bar.

When a widget is in the above state, resizing the main window fails to resize any widgets contained within it. This failure to resize widgets has been intermittently observed on the Mac as well.