Twenty Nineteen: heading style breaks headings in content added by core or plugins in the front end

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The Twenty Nineteen style for headings uses some CSS generated content to draw a short horizontal line before the headings.

This comes from the post-section-dash mixin used for various elements, for example the h1-h2 headings, entry-title, author-title, and comments-title.

So far so good. But when "external" content is added to a Twenty Nineteen page, the horizontal line CSS rule targets any H1 and H2 headings.

For example, when adding the media modal in the front end, the modal headings break. See attached screenshots.

This also applies to any content added by plugins when the added content includes H1 and H2 headings.

Seems to me this CSS selectors should be better scoped to try to target only the headings within the theme content:

​https://github.com/WordPress/twentynineteen/blob/b604f127c2cae10bd48bbbec0fbbbff2cd31f957/sass/elements/_elements.scss#L55-L58

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"select" media modal used in the front end

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"add media" modal used in the front end