Dialog windows don’t mark mouse actions as handled for the subjacent environment.
If we e. g. operate the “Change appearance” window in design mode and select a character, the engine additionally may also select an actor of the subjacent map (if there is one by chance). This is possible without another mouse click.
We get the same behavior with the “Properties at Startup”-section when calling the “When-touched”-event of an actor. This only is reproducible for actors with a longer property list. The mouse wheel will not only scroll the screenplay but also the property section, whenever the mouse pointer hovers over its range. You’ll notice that after closing the screenplay window.