If you edit an article and have changed anything and close the Browser-Window/Tab or click the "cancel"-button a warning that you will lose your edit will be displayed.
This should also be the case for TwoColConflict-Pages.
It seems that the "Cancel" button correctly prompts for unsaved changes, and after you've done that, closing the tab also correctly prompts. The bug only appears if you close the tab before clicking any of the controls.