Content width in block editor styles expects padding

[SergeyKovalets]

When using the block editor, the content width while creating/editing a post is different from what is displayed after publishing on the site. When creating a post, I can't be sure how it will look once published.

If I use the classic editor, everything is fine — the way it looks while creating is exactly how it looks after publishing.

[sabernhardt]

All themes from Twenty Ten to Twenty Seventeen added 30 pixels to the width of .wp-block 'to account for padding' in [44152]. WordPress 5.4 removed that padding, but the number was already inaccurate with WP 5.0. (Left and right padding were set to 14 pixels for small screens, or 43 pixels each for larger screens.)

To accommodate older versions of WordPress, Twenty Fourteen probably could set the width like this in editor-blocks.css:

.wp-block {
	max-width: 474px;
}

.wp-block:where(.editor-post-title__block, .editor-block-list__block, .editor-default-block-appender) {
	max-width: 504px; /* account for padding in older WordPress versions */
}