Fix bump in specificity for layout styles in non-iframed editor

[talldan]

Trac ticket: https://core.trac.wordpress.org/ticket/61829
Github PR: WordPress/gutenberg#64076

Backports the changes from WordPress/gutenberg#64076

What?

Fixes the issue mentioned in WordPress/gutenberg#53468 (comment) to restore theme.json spacing rules taking precedence over the implicit spacing rules in a non-iframed editor.

How?

In trunk the code generates a selector like .is-layout-flow > * (specificity 0,1,0) which is fine normally. When in a non-iframed editor, the transformStyles function prefixes selectors with .editor-styles-wrapper to scope them to the editor canvas, so it becomes .editor-styles-wrapper .is-layout-flow > * (specificity 0,2,0), a bump in specificity that causes the precedence of styles to change.

In this PR I've changed the selector to :root :where(.is-layout-flow) > * (still specificity 0,1,0). transformStyles handles 'root' selectors a little differently, it'll instead replace the :root part so it becomes .editor-styles-wrapper where(.is-layout-flow) > * (keeping the specificity at 0,1,0).

The other layout selector that this affects is the :first-child :last-child selectors that are responsible for resetting margin at the start and end of a block list. They traditionally have a 0,2,0 specificity so that they can override both the above rule and any rules in the theme.json. Those selectors are also maintained at 0,2,0 with this change, they become something like :root :where(.is-layout-flow) > :first-child.

Here's a list of selectors that will be updated (according to our php tests)

-.is-layout-flow  > :first-child {
+:root :where(.is-layout-flow) > :first-child {

-.is-layout-flow  > :last-child {
+:root :where(.is-layout-flow) > :last-child {

-.is-layout-flow  > * {
+:root :where(.is-layout-flow) > * {

-.is-layout-constrained  > :first-child {
+:root :where(.is-layout-constrained) > :first-child {

-.is-layout-constrained  > :last-child {
+:root :where(.is-layout-constrained) > :last-child {

-.is-layout-constrained  > * {
+:root :where(.is-layout-constrained) > * {

-.is-layout-flex  {
+:root :where(.is-layout-flex) {

-.is-layout-grid  {
+:root :where(.is-layout-grid) {

-.wp-block-social-links-is-layout-flow  > :first-child {
+:root :where(.wp-block-social-links-is-layout-flow) > :first-child {

-.wp-block-social-links-is-layout-flow  > :last-child {
+:root :where(.wp-block-social-links-is-layout-flow) > :last-child {

-.wp-block-social-links-is-layout-flow  > * {
+:root :where(.wp-block-social-links-is-layout-flow) > * {

-.wp-block-social-links-is-layout-constrained  > :first-child {
+:root :where(.wp-block-social-links-is-layout-constrained) > :first-child {

-.wp-block-social-links-is-layout-constrained  > :last-child {
+:root :where(.wp-block-social-links-is-layout-constrained) > :last-child {

-.wp-block-social-links-is-layout-constrained  > * {
+:root :where(.wp-block-social-links-is-layout-constrained) > * {

-.wp-block-social-links-is-layout-flex  {
+:root :where(.wp-block-social-links-is-layout-flex) {

-.wp-block-social-links-is-layout-grid  {
+:root :where(.wp-block-social-links-is-layout-grid) {

-.wp-block-buttons-is-layout-flow  > :first-child {
+:root :where(.wp-block-buttons-is-layout-flow) > :first-child {

-.wp-block-buttons-is-layout-flow  > :last-child {
+:root :where(.wp-block-buttons-is-layout-flow) > :last-child {

-.wp-block-buttons-is-layout-flow  > * {
+:root :where(.wp-block-buttons-is-layout-flow) > * {

-.wp-block-buttons-is-layout-constrained  > :first-child {
+:root :where(.wp-block-buttons-is-layout-constrained) > :first-child {

-.wp-block-buttons-is-layout-constrained  > :last-child {
+:root :where(.wp-block-buttons-is-layout-constrained) > :last-child {

-.wp-block-buttons-is-layout-constrained  > * {
+:root :where(.wp-block-buttons-is-layout-constrained) > * {

-.wp-block-buttons-is-layout-flex  {
+:root :where(.wp-block-buttons-is-layout-flex) {

-.wp-block-buttons-is-layout-grid  {
+:root :where(.wp-block-buttons-is-layout-grid) {

Testing Instructions

1. Add some very obvious margin rules to your theme.json like this for paragraphs (under styles.blocks):

"core/paragraph": {
	"spacing": {
		"margin": {
			"top": "4rem",
			"bottom": "4rem"
		}
	}
},

2. Create a new post. Ensure you don't have custom fields active.
3. Insert 3 paragraphs with text, check that the margin looks right. The first paragraph should have no top margin, the last no bottom margin, but in all other cases there should be 4rem top/bottom margin.
4. Activate Custom Fields in the editor preferences to 'un-iframe' the editor
5. Check that the styles are the same without the iframe

Also worth checking out trunk and comparing the styles. You should see that in trunk the iframed editor looks correct, but when you activate custom fields the non-iframed editor styles are broken.

Screenshots or screencast

Before

After

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[aaronrobertshaw]

Thanks for putting this together @talldan 👍

I've tested it in the post editor, following the instructions on this PR and the additional steps outlined over on the Gutenberg PR in WordPress/gutenberg#64076 (comment).

✅ Block-level margin styles in global styles overrides base spacing styles
✅ Block-level margin styles in global styles overrides block level spacing in global styles (e.g. Group block set to 0 block spacing will be overridden by paragraphs within the Group if paragraphs have margin set)
✅ Block instance level block spacing overrides global styles margin rules
✅ Block instance level margins override block instance level block spacing
✅ Block spacing for buttons, social icons, row, stack, and grid all appear to be working

I didn't spot any issues. This LGTM 🚢

Before (iframed)	Before (non-iframed)

After (iframed)	After (non-iframed)

[hellofromtonya]

• ✅ Resolves the issue - See my test report.
• ✅ Correctly generates the expected CSS - See my findings report.
• ✅ Changes match changes in Gutenberg PR.

LGTM and ready for commit.

[hellofromtonya]

Committed to trunk via https://core.trac.wordpress.org/changeset/58890.