It turns out that Unicode support in Terminals is a lot more difficult than it first appears. A quick overview of special support for Unicode characters in Terminals:

• "Wide" or "Fullwidth" characters, particularly for East Asian languages and emojis, are codepoints that occupy two cells in a terminal instead of one.
• "Zero" width combining characters used in languages such as Arabic, Hebrew, or Hindi do not occupy any cells themselves; instead, they modify the previous character.
• "Zero Width Joiner" (ZWJ U+200D) reduces and combines many codepoints into a single emoji. This is similar to combining, but encoded in a completely different way.
• "Variation Selector-16" (VS-16 U+FE0F) is a special character that, for specific "Narrow" emojis consuming one cell, causes them to become "Wide", consuming two cells.

Reddit thread where the Textual Markdown widget was announced.

Also includes the rather hilarious comment "I wish there were more full blown app examples out there", back when Textual was pretty damn new and barely out of the initial "CSS branch" release.