"Unload event listeners are deprecated" browser error

[Óscar Frías Barranco]

Hello.

Browsers running our GWT application (version 2.11) have recently (this week) started reporting errors like this one:

[{"age":41550,"body":{"columnNumber":192,"id":"UnloadHandler","lineNumber":3210,"message":"Unload event listeners are deprecated and will be removed.","sourceFile":"ourmodule-0.js"},"type":"deprecation","url":"oururl","user_agent":"Mozilla/5.0 (Linux; Android 10; K) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/121.0.0.0 Mobile Safari/537.36"}]

Any idea about why is this happening?  Is GWT using a deprecated browser feature?

Thanks!

Oscar

[Thomas Broyer]

Yes: https://developer.mozilla.org/en-US/docs/Web/API/Window/unload_event

Apparently, this happens when you use Window.addXxxHandler (and for instance the default PlaceController's delegate calls Window.addClosingHandler, which registers an unload handler but is only interested in the beforeunload event)

[Colin Alworth]

It looks like the purpose of registering the unload handler for any Window event is to avoid IE6-10 era memory leaks. From the notes on the MDN page, removing those other handlers will break the page when the bfcache is in use, but our use of the unload handler will opt GWT pages out.

The simplest fix appears to be to stop initializing the unload event tracking for resize/scroll events, and "let" those leak (which they won't, because IE in all forms is really-actually-totally-dead this time, right?). Next unload/beforeunload should be decoupled, so that each can be initialized separately. Then applications can remove their own usage of unload as they wish, but we should probably deprecate this event handler method (not the event and handler itself) as well.

I have filed https://github.com/gwtproject/gwt/issues/9908 to track this.