Beforeunload: ScriptDialogOpening's deferral and/or args.Accept() is useless

[TomPMoleman]

According to the official documentation of the 'ScriptDialogOpening' event callback, the JS invocation shall be blocked in case you are requesting the deferral.
However, I noticed, that the deferral is completely useless for the 'beforeunload' kind.

Steps to reproduce:

• declare 'AreDefaultScriptDialogsEnabled' setting false in code
• bind ScriptDialogOpening event callback in code
• navigate to very basic sample page: https://www.w3schools.com/jsref/tryit.asp?filename=tryjsref_onbeforeunload
• click the link

Error behaviour:
Please note, that the ScriptDialogOpening event callback will be invoked, but regardless of fetching the deferral and/or invoking the Accept() function on the event args, the navigation will always take place.

Expected CORRECT behaviour:
The default JS dialog (shown upon setting 'AreDefaultScriptDialogsEnabled' to true) is showing a dialog with two options to either 'stay on the page', or to 'leave the page' and depending on the selection the navigation is either executed or aborted.
=> However, with the described error behaviour above in place, writing one's own code to reflect these two options won't be feasible (since the 'leave page' scenario will inevitably be the only outcome).

How to fix:
Just as for the other 3 kinds (alert, confirm & prompt) also for 'beforeunload', within the ScriptDialogOpening event callback, the deferral should block further execution and depending on calling the 'Accept()' function on the event args, the 'leave page' (accept was triggered) and 'stay on page' (accept was NOT triggered) options shall be reflected.

Affected version: 1.0.1185.39 (as used within Windows App SDK version 1.2.230217.4)

AB#44218030

[monica-ch]

@TomPMoleman Thanks for the bug report. We have fixed the issue in Edge canary 115.0.1830.0+. Please validate and let us know if you have more questions.