Includes the following fixes:
- [Block Library]: Less warnings when blocks try to render themselves.
- Reset z-index on focused widget form
- Refactor appender margin.
- Fix slash inserter for widgets screen.
- Widget screens: set html block as freeform content handler.
- Widget Block: widget_id is undefined when a widget is placed.
- Add <CopyHandler> to WidgetAreasBlockEditorProvider
- Add width: 100% to components-base-control inside wp-block-legacy-widget
- [Widgets editor] Replace the "technical" error notice a more user-friendly one
- Fix legacy widget height overflow
- Fix "Select all" behavior in the editor
- Increase specificity of the NoPreview CSS rules to avoid conflicts with theme styles
- Fix move to widget area checkmark
- Replace legacy widget icon with its new version
- [Block Library - Query Loop]: Set default block variations not to inherit from global query
Merges [51344] to the 5.8 branch.
Fixes#53397.
Built from https://develop.svn.wordpress.org/branches/5.8@51346
git-svn-id: http://core.svn.wordpress.org/branches/5.8@50955 1a063a9b-81f0-0310-95a4-ce76da25c4cd
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