Widget Editor: Function-based widgets don't render and don't save

[adamziel]

Description

The widgets editor doesn't play nicely with a function-based marquee widget. Presumably, this is a problem also for other function-based ("reference") widgets.

Step-by-step reproduction instructions

1. Add the marquee widget to your WP installation
2. Go to the widgets editor and add marquee
3. Try to save the form

Expected behaviour

1. Everything gets saved buttery smooth.
2. The Marquee widget shows up in the "inactive" panel as it used to in the old editor:

Actual behaviour

1. The form doesn't render. Then it doesn't get saved. The marquee isn't previewed. Saving the entire widgets screen produces a notice with an error.
2. The widget can't be seen in the "inactive" panel

Screenshots or screen recording (optional)

Nagranie.z.ekranu.2021-06-24.o.16.05.45.mov

[adamziel]

Once I opted out of the block-based widgets editor with add_filter( 'use_widgets_block_editor', '__return_false' );, I saw the marquee widget in the "Inactive" section of the old widgets editor. From that moment on, it also started showing up in the new editor (wp_get_sidebars_widgets() started returning marquee_greeting when it wouldn't previously). There must be a write operation somewhere in the legacy editor that adds such "missing" widgets to the inactive section.

[getdave]

function-based marquee widget

What is a "function based widget"? Looking at https://gist.github.com/adamziel/526436b4cc9bc82a7221c77e00137ae1 I assume it's literally a Widget that is created using a function rather than a class?

[adamziel]

@getdave you got it right! There's a bunch of logic specific to this kind of widgets in WordPress core.

After resetting my wp-env, the marquee greetings showed up in the "Available widgets" section of the legacy editor, not in the "inactive widgets" section 🤔

It was picked up from the $wp_registered_widgets global.

[adamziel]

One problem is how the marquee widget triggers a request to /wp/v2/widgets/marquee_greeting where we assume that is_null( $sidebar_id ) means there is no widget with a given ID and return 404:

gutenberg/lib/class-wp-rest-widgets-controller.php

Lines 155 to 161 in 7733153

	if ( is_null( $sidebar_id ) ) {
	return new WP_Error(
	'rest_widget_not_found',
	__( 'No widget was found with that id.', 'gutenberg' ),
	array( 'status' => 404 )
	);
	}

This assumption holds for widgets implemented using classes