Navigation: restore flex-grow on container when accessible label is present

packages/block-library/CHANGELOG.md

@@ -10,6 +10,7 @@
 
 ### Bug Fixes
 
+-   Navigation: Restore `flex-grow` on the menu container in the editor, where the visually hidden menu description breaks the `:only-child` selector the front end relies on, so the "Space between" and other justification settings apply in the canvas as they do on the front end ([#78447](https://github.com/WordPress/gutenberg/pull/78447)).
 -   Image: Fix cropped galleries rendering images at their natural height in the editor canvas. The baseline inline `height: auto` is no longer emitted for images inside a cropped gallery, so the gallery's own cropping CSS applies ([#82318](https://github.com/WordPress/gutenberg/pull/82318)).
 -   Tabs: Activate the tab that a URL hash points into, so an anchor set on a block inside a tab panel can be reached. Anchor links followed after the page has loaded are handled too, matching the Accordion block ([#81744](https://github.com/WordPress/gutenberg/pull/81744)).
 -   Accordion Panel: Reset padding-block when panel is hidden ([#81782](https://github.com/WordPress/gutenberg/pull/81782)).

packages/block-library/src/navigation/editor.scss

@@ -76,6 +76,15 @@
 	}
 }
 
+// The AccessibleMenuDescription component renders a visually-hidden element as
+// a DOM sibling of the navigation container. This breaks the :only-child selector
+// in style.scss that grants flex-grow: 1, causing space-between layout to have
+// no room to distribute items in the editor. Restore flex-grow via the
+// editor-only class that InnerBlocks adds to the container.
+.wp-block-navigation__container.block-editor-block-list__layout {
+	flex-grow: 1;
+}
+
 // IE fix for submenu visibility on parent focus.
 .is-editing > .wp-block-navigation__container {
 	visibility: visible;