This question is an exact duplicate of:
When I'm making an edit I prefer to have the original post visible. This is so I can verify that I am improving on the original, not introducing errors and preserving the original meaning of the post. I haven't been able to identify any features which would allow me to view the original post while I'm editing. Therefore, when I edit, I typically open a new tab with the original question to help me verify the quality of my changes. Given that the site's policy on edits is that they be substantial, I find it difficult to keep all of the changes in my memory.
I think some way to differentiate between what I've added and what the post currently contains is necessary. Is it that a feature addressing the problem is available and I just haven't been able to find it? If this feature doesn't exist I would propose that the original post be displayed in the same manner that questions are displayed while writing an answer.
I'm interested in reading how other members currently ensure their edits improve on a post when the original post is not immediately visible.