Posted by Henrik Sperre Johansen on Mar 02, 2011; 9:34am
URL: https://forum.world.st/Assert-SystemWindow-not-visible-tp3330492p3331230.html

Also note, in 1.2 (I think) onwards, you can subscribe to WindowAnnoncements.
WindowClosed should be announced when the window closes, so something like:
|window closed|
closed := false.
window := SystemWindow labelled: 'Zork'.
window announcer when: WindowClosed do: [closed := false].
window openInWorld delete.
self assert: closed.

should work.

Cheers,
Henry