When typing a comment with a code snippet (as a fenced block in triple backticks) for a pull request, the preview displays at the width the snippet needs. There is no scrolling (like in the real comment).

This can be a problem if the list of changed files in the right column is long, as the overflow preview is hidden under it including the Preview button that gets you back to editing.

In the screenshot the list is just long enough to hide the button. If it were longer, it would cover the preview as well.

Maybe the preview should be horizontally scrollable just like the comment will be once posted.