Consider deprecating "function widgets"

[adamziel]

Description

This issue is all about function widgets like the Marquee Widget. They are a special case of widgets and the new editor has a few problems with handling them:

1. Changing a form field of a function widget doesn't "activate" the "Update" button
2. Saving the widgets screen using the "Update" button does not preserve changes from the function widget form

1. The function widgets tend to get added to the "Inactive widgets" section seemingly randomly, if the user doesn't notice they won't be able to add another one to their desired sidebar.
2. ...I am willing to bet there's more. We already adressed a bunch, e.g. Widget Editor: Function-based widgets don't render and don't save #32960, Fix flaky widgets-related E2E tests #33066, Widgets: Fix creating and editing non-multi widgets #32978

There are two ways of approaching these bugs:

1. Whack-a-mole every problem as we uncover them
2. Deprecate the concept of a "function widget", eliminate all the code branches that are specific to function widgets, and wrap them in a class using code similar to this (props to @noisysocks):

function wp_register_sidebar_widget( $fn ) {
	register_widget( new class extends WP_Widget {
		public function widget() {
			call_user_func( $fn );
		}
	} );
}

The latter approach has some limitations. For example, there’s an implicit assumption that there’s just one instance of any function widget, which means that adding more than one of them and clicking the "Update" button would effectively update the same database entry multiple times. On the other hand, we could add $is_multi = false to such WP_Widget to simulate that behaviour in a less weird way. On yet another hand, we could just wear the weirdness and say “it’s deprecated, tread carefully!” in some place in the UI.

That being said, I am a big proponent of deprecating function widgets and would like to discuss it here.

cc @noisysocks @draganescu @talldan @kevin940726 @gziolo @spacedmonkey @ellatrix @hellofromtonya @getdave

[noisysocks]

Thanks for creating an issue to discuss and track. I'd very much like to deprecate this kind of widget after WordPress 5.8. I think also it might make sense to do this as part of refactoring widgets to use CPTs as the storage mechanism (see Core-35669).

[noisysocks]

Just quickly noting:

Changing a form field of a function widget doesn't "activate" the "Update" button

This is expected. Function widgets bypass the normal Update flow because their data can be stored somewhere that we don't know about. That's why they have their own Save button.

The function widgets tend to get added to the "Inactive widgets" section seemingly randomly, if the user doesn't notice they won't be able to add another one to their desired sidebar.

Also expected. You can't delete a function widget and so if you try it will re-appear in Inactive widgets. I agree it's not obvious that this is happening.

Also worth noting is that these types of widgets are incredibly uncommon 🙂

[adamziel]

This is expected. Function widgets bypass the normal Update flow because their data can be stored somewhere that we don't know about. That's why they have their own Save button.

If I wear my developer hat, I can understand that. If I wear my user hat, I am confused why sometimes there's another save button and why updating things doesn't always update things.

Also worth noting is that these types of widgets are incredibly uncommon 🙂

That's a good news! It should make it easier to deprecate that then.

[adamziel]

Let's continue in https://core.trac.wordpress.org/ticket/53993