Steps to see what I mean:
1. Open a 2-way file comparison.
2. Choose File > Compare Current File.
3. Add some new-lines (or make some other change) to the file in the new editor that opens.
4. Save the file in that editor.
5. Return to the original file comparison.
The changes to the file in the other editor are not shown and if the file is saved from the original comparison, the changes made in the other editor will be lost.

Real world use case was having a file opened in two comparisons, one with its previous revision, and another comparison with a similar file with similar changes already updated in the repository. I know a three-way comparison could have been used but that's not how I was thinking about it at the time.