centralWidget and QDockWidgets in designer

[jpn]

Try adding the dock widget by the time when the top level layout has not yet been set. Otherwise the dock widget seems to end up into the layout. Change the dock widget's properties (docked, dockWidgetArea) after dropping it on the form (docked property comes visible only when the dock widget is not in a layout and dockWidgetArea property comes visible after setting the docked property to true).

About the central widget, Designer handles it like you suspect. The central widget is created automatically and the top level layout is set on the central widget.