onbeforeunload not handled when closing tab

[petemill]

Description

When window.onbeforeunload is set, and it returns a string, the tab should not be closed unless the user confirms a prompt. This does not happen on Brave. #4079 states it is fixed, but it does not work as intended in either 0.19.x or 0.21.x, for tab close events. It does, however, work when refreshing or navigating away in the same tab.

Steps to Reproduce

1. Visit http://petemill-code-samples.s3-website-us-west-1.amazonaws.com/onbeforeunload/
2. Focus the tab content by clicking inside the content area
3. Close the tab

Actual result:
Tab closes

Expected result:
A prompt should be shown confirming if the user really wants to close the tab.

Reproduces how often: 100%

Reproducible on current live release:
Yes

Additional Information

Chrome, Safari and Firefox all present dialogs with 'Leave' and 'Stay' buttons.

[bsclifton]

codepen needs update

[petemill]

Sample updated @bsclifton http://petemill-code-samples.s3-website-us-west-1.amazonaws.com/onbeforeunload/

[petemill]

FYI it seems like the way that other browsers protect against malicious sites defining an 'alert' trail by traversing multiple sites which all define onbeforeunload handlers, is to require that some user focus be actioned inside a frame before allowing the handler to be registered.