This question already has an answer here:
In the first post queue, I see a question where the asker posted important supplemental information in two comments. So I click “edit”, and the comments disappear: all that's left is the standalone editor, with no link to the post, no comments, nothing but the title, body and tag edit boxes.
To get the comment text, I need to open the same URL in a different tab, and copy the comments across tabs. That's not as efficient as it could and should be.
The same issue arises when suggesting an edit from the post itself: the out-of-page editor doesn't show the comments, and doesn't have a link to the page. Same thing when reviewing suggested edits.
Please always show the comments while editing a post.