At screen widths of at least 768px (tablet size), Twenty Nineteen should have a `max-width` and side margins for content in the post editor.
This commit applies that style to the new `.block-editor-iframe__body` class, resolving an issue where full-width blocks had a horizontal scrollbar in the editor iframe.
Props sabernhardt, smit08, nidhidhandhukiya, mukesh27, darshitrajyaguru97, oglekler, poena, shailu25, nicolefurlan.
Fixes#59449.
Built from https://develop.svn.wordpress.org/trunk@56832
git-svn-id: http://core.svn.wordpress.org/trunk@56344 1a063a9b-81f0-0310-95a4-ce76da25c4cd
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