I found a user interface inconsistency between mobile and desktop view.
When not a member of a community and visiting (for one specific example) Law.SE, using Firefox Android 63.0b3 with "Request Desktop Site" unchecked, the "add a comment" link appears active - but clicking on it seems to do nothing, though there is a bit of flickering on the bottom of the screen which dismisses itself (you can continue clicking on "add a comment").
If instead you check the "Request Desktop Site" checkbox the "add a comment" link isn't as brightly highlighted but is still clickable. In the desktop mode clicking produces a helpful message mentioning that you need to join the community and have 50 reputation to comment.
2 related bugs seem to be:
In mobile view mode: The highlighting for "add a comment" isn't the same color/brightness as, for example, "improve this answer" (that link would operate without joining) - it's brighter and appears as though it could work, but there's no popup (maybe one mostly offscreen at the bottom of the page which crashes/dismisses itself). The user experience makes it seem like the link is broken.
In desktop view mode: The highlighting for "add a comment" is the same color/brightness as, for example, "improve this answer" (that link would operate without joining) - even though it appears on equal footing (same color/brightness) as other links it isn't usable (which is correct) and unlike the mobile view a helpful popup reminds the user what the problem is. The user experience is better than the mobile view, perhaps it could be lighter colored; possibly it's fine as-is if there's no interest in lightening the link's color.
Short version: Mobile view popup broken on comment's links, highlighting of usable links for desktop versus mobile view should be more consistent.
Another point for a seperate bug report is: The mobile version doesn't display the 'new user banner' over the answer box like the desktop version does.