Add editor styles to the widgets block editor

[isabel_brison]

Initially reported here: ​https://github.com/WordPress/gutenberg/issues/32449

(style changes are visible in the front end, but not previewable in the editor)

The settings passed to the widgets editor initialization function don't include theme styles. In the post editor, theme styles are being added to the settings here: ​https://github.com/WordPress/wordpress-develop/blob/master/src/wp-admin/edit-form-blocks.php#L131-L132 so perhaps we should do the same for Widgets.

[noisysocks]

Closing this in favour of ​https://github.com/WordPress/gutenberg/issues/26163.

[isabel_brison]

Closing this in favour of ​https://github.com/WordPress/gutenberg/issues/26163.

If we do go ahead with allowing theme styles in the Widgets editor, the change will have to be done in core, so we should probably close the Gutenberg ticket in favour of this one instead.

[isabel_brison]

Reopening this to try adding theme styles in the Widgets editor.

Trac ticket: https://core.trac.wordpress.org/ticket/53344

Loads theme styles in the Widgets editor in the same way as they are loaded in the post editor. Will also require some CSS changes on the Gutenberg side, but basic functionality can already be tested in this PR by enabling Twenty Twenty One and checking that its styles are loaded in the Widgets editor.

Oh, could you add a unit test for get_block_editor_theme_styles? You can use the the one we have for get_legacy_widget_block_editor_settings as inspiration.

[noisysocks]

#53388 was marked as a duplicate.

Test added!

[desrosj]

Reading a milestone for tracking.

[audrasjb]

#53454 was marked as a duplicate.

[audrasjb]

I tested the patch in TT1 and it fixes the default font-family issue I reported in #53454, which was a duplicate of this ticket :)

Awesome. LGTM 👍

[desrosj]

I added some questions in a review on the PR. But overall it looks really good and tests well! There are some small styling adjustments that will need to be made in the default themes, but the widgets at least stylistically match the theme now.

Also, it looks like ​WordPress/gutenberg#32683 was merged. Does this have to land after the pacakge updates are committed to Core? Or can this be committed at any time now?

This can be committed any time! If the package updates aren't in yet we'll see some styling bugs, but no major breakage.

[desrosj]

In 51200:

Widgets: Add editor styles to the widgets block editor.

This updates the widgets screen to load the editor styles in the same way as the post editor. This combined with the package updates in [51198] and [51199] ensures that the blocks added to sidebars more accurately reflect what will be displayed on the front end of the site.

Props isabel_brison, noisysocks, andraganescu, audrasjb, jorbin, caseymilne, desrosj.
Fixes #53344. See #53388.

Thanks everyone! Merged into Core in https://core.trac.wordpress.org/changeset/51200.