[Webkit-unassigned] [Bug 255837] New: Calling `event.preventDefault()` for the `beforeunload` event does prevent the page from unloading.

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=255837

            Bug ID: 255837
           Summary: Calling `event.preventDefault()` for the
                    `beforeunload` event does prevent the page from
                    unloading.
           Product: WebKit
           Version: Safari 16
          Hardware: All
                OS: All
            Status: NEW
          Severity: Blocker
          Priority: P2
         Component: Page Loading
          Assignee: webkit-unassigned at lists.webkit.org
          Reporter: chewteeming01 at gmail.com
                CC: beidson at apple.com

Created attachment 466050

  --> https://bugs.webkit.org/attachment.cgi?id=466050&action=review

The HTML file contains a link that should be intercepted by the beforeunload event.

Summary:
Calling `event.preventDefault()` when listening to a `beforeunload` event on the `window` should rightly prompt the user if they want to stay or leave the page. This is currently broken in macOS Safari 16.4.

OS:
I have tested this issue on both iOS Safari 16.4.1 and macOS Safari Version 16.4 (18615.1.26.11.23).

Steps to reproduce:
An example page such as https://www.w3schools.com/jsref/tryit.asp?filename=tryjsref_onbeforeunload will not work on iOS, but will still work on macOS (it only works on macOS because the unload event is contained in an iframe.)
I have attached a sample HTML file that can be opened in a Webkit browser. Clicking on the link should prompt the user with an alert. The code was copied from the example provided by W3Schools.

Actual results:
The browser navigates to the link normally.

Expected results:
The browser prompts the user with an alert to stay or leave the page before navigating to the new page.

Build Date & Hardware:
Apple M1 Pro
macOS 13.3.1 (22E261)

Additional information:
I first noticed this issue during some cross-browser tests for cancelling the `beforeunload` event. Only Safari was failing the tests at the time so I started to investigate. The PR with those cross-browser tests can be found here: https://github.com/sveltejs/kit/pull/9747

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