Are you referring to this table here, which already exists or is there another problem you're trying to solve. It's unclear if you just have a question on how to use Phabricator or are asking for that workflow to be more efficient.

(We don't take UI feedback as feature requests, for the record).

I think the author is essentially asking for an easy way to map inline comments to the diff that they were originally made upon.

Sorry, to clarify, I'm just asking for a convenience feature in the UI. I'm aware of the History tab, but I find that I often specifically want to see changes made since a certain comment (e.g. to check if/how the issue was addressed). In other code review software I've used, this workflow only takes a single click.

I don't think this is about inline comments, since @ajklen says "comment dropdown menu" and inline comments don't have one. I believe they want this, in the existing dropdown menu of (non-inline) comments:

+---+
| V |
+---+------------------------------------------+
|  " Quote Comment                             |
| </ > View Remarkup                           |
| - - - - - - - - - - - - - - - - - - - - - -  |
|  X  Remove Comment                           |
| - - - - - - - - - - - - - - - - - - - - - -  |
| ~~~ View Changes Since This Comment Was Made | <-- New Option
+----------------------------------------------+

Selecting this option would be equivalent to going to the History tab, selecting the active diff at the time of the comment on the left, selecting the current diff on the right, and clicking Show Diff.

(That said, it's still not clear to me why this is useful/desirable or what problem it solves.)