Add ToggleEvent source

[tunetheweb]

Fixes #3604

Has quite different support than the rest of Toggle Event so added it as a new feature:

[ddbeck]

Thanks for starting this, @tunetheweb!

[ddbeck]

If I understand correctly, the toggle event fires for <details>, <dialog>, and popovers. But I'm not sure how to understand the spec with the source property. Is the property only for popovers? I think the answer to this makes a big difference for how we frame this feature.

[ddbeck]

Thinking on this a little more, can you point to an example of how this might be used? The MDN reference page example feels a little contrived and I'm trying to figure out what a developer might do with this that they could not without this property.

[tunetheweb]

If I understand correctly, the toggle event fires for <details>, <dialog>, and popovers. But I'm not sure how to understand the spec with the source property. Is the property only for popovers? I think the answer to this makes a big difference for how we frame this feature.

Oh you're right it's for more than just Popovers (though that's a big use case). Updated to drop the "Popover" from the feature name.

And here's an example of how it might be used:
https://chrome.dev/css-wrapped-2025/#toggleevent-source
(which coincidently is why we want to add this feature so we can add a nice Baseline widget to this page 😁)

[ddbeck]

Thanks for the link to the Wrapped page—something about the "responsible" element cleared this up for me. Some little things to fix (most importantly, this feature's ID), but this is close to merging. Thank you!

[ddbeck]

Thank you!