Some Markdown files (e.g., README.md) are automagically rendered into HTML. However, there’s no obvious way to view, let alone edit the source code. Contrariwise, files such as markdown.md.erb are presented as source code and have no link for viewing the rendered version. Here are some example links:
Between the last commit information and the HTML version of the file is a small row containing the file name, and a few buttons (“Blame”, “History”, and “Permalink”). Next to the buttons is a small icon of a page with an HTML tag inside (screenshot below). Hovering over this will show the tooltip “Open Raw”, and clicking it will show the markdown source.
To my knowledge, Gitlab will not preview the erb files to avoid any issues caused by running the Ruby inside them.
It turns out that the row of names and buttons (along with some other information) is hidden if the window is too narrow. Perhaps this content should be moved down, rather than hidden, in this case.
I realize that running arbitrary Ruby code can present security issues, but it could also be quite useful. For example, I make heavy use of variables to tidy up my markup, present dynamic content, etc. More to the point, why does the page in question (markdown.md.erb) document something that isn’t actually functional?