Enable or disable all top-level windows. This allows nesting. In other words, calling EnableMainWindows(false) twice requires 2 calls to EnableMainWindows(true) before the top level windows are actually enabled. An example of where not allowing nesting was a problem was the Tools/Options dialog, which could open a PossListChooser dialog. Before, when you closed the PossListChooser, you could select the main window.