This is not quite it, though. The workflow diff allows you to compare your local change to the remote branch before checking in your changes.
This means you can only compare the changes you made to how it was before. What OP would like (and me too, for that matter), is an easy way to check a pull request where a completed branch needs to be merged into the master. Currently you only see which files have changed (and in Azure which lines are different but that’s still in XAML syntax and hard to read, especially for more complex changes).
Ideally workflow diff would work by comparing the branch I am on to a branch of my choice (e.g. Master), not just its own upstream branch before I commit my changes.