This is more of an user-interaction edge case than a bug:
When reading a long markdown document in Preview, there are relative links to text files, e.g. [Another File](/reference-file.txt)
. Click the link to the relative file and it displays in preview as black monospaced plain text on white. In order to go 'back' to the originating preview you must go back to the editor, make a change, then return to Preview for the refreshed markdown preview of the initial document. (Note: a change must be made to revert the preview to the original document. Otherwise Preview is stuck on the relative file in plain-text mode.)
Possible solution: a 'back' button that appears when you open a second document in preview. An added bonus: showing the referenced text file as a rendered preview.