Setext-style header's underlining dashes don't fully covering the text

[legends2k]

Expected Behavior

When using setext-style (underlining) header, the underlining dashes (- or =) is generally of the same count as the text underlined e.g.

Heading 1
=========

Heading 2
---------

This is eligible when reading i.e. the underline runs the whole span of the text. When authoring it's straight forward so to the number of dashes to key in.

Actual Behavior

markdown-mode shows the dashes in a size different/smaller from the heading's size. As a result, though the dash count is the same as the heading text count, it looks as though the dashes aren't enough to completely underscore the heading text. Likewise, when writing a markdown, the number of dashes to type is not readily evident.

Of course, this isn't reproducible on a terminal or when running windowed with no theme. Only when a non-trivial theme is loaded does this issue show-up.

I know that one solution to this problem would be use a theme that doesn't show different heading levels in different sizes. However, that's a nice feature to have.

Another fix, that feels more optimal to me, would be to set the font size of the underlining dashes also to be the same as the heading text's. The dash colour can continue to be some washed out colour like grey. Though the actual colour and size would be coming from the theme, markdown-mode can set the dash's kind of the face to be same as the heading's.

Software Versions

• Markdown Mode: 2.4-dev
• Emacs: 26.2
• OS: macOS Mojave