I'm a bit confused by this and the posts above. In vscode it is possible to configure it such that on every save a new pdf is generated. You can open the PDF to the right and it's essentially instant preview.

I've used this config when writing papers, etc, and it works flawlessly.

As for diffs (what has changed), it is also possible to color code the changes too.

I can't recall how I configured viscose, but it was a fairly standard approach.