Window within ExtendSim does not appear even though the window is active and should be visible.
This problem can occur when working on multiple monitors, then switching from multiple monitors to a single monitor. The "hidden but open" window has actually opened outside the boundaries of the current ExtendSim application. The model window is remembering its previous location when ExtendSim had broader boundaries on the additional monitor.
To find windows that have disappeared from view, either:
From within ExtendSim, go to the menu command Window > Tile or Window > Cascade. Both Tile and Cascade will display all your open windows within your ExtendSim window.