The on_stabilized() callback

          const x::w::busy &mcguffin)
             // ...

The on_stabilized() callback executes the lambda after the window's size and position gets computed and the window becomes visible. The window's final size and position is not fixed until it's visible. The display's window manager has the final word on a new window's position and size, and this cannot be determined in advance.

toolboxlayoutmanager.C shows both the main window and its toolbox dialog initially. Since the toolbox dialog's default position should be to the left of its window, toolboxlayoutmanager.C needs to wait to show the dialog until its main window is stable.