Question: Suppose I am working on a long edit session of a previously existing file and have not saved the edits/changes yet. I would like to compare this changed, yet unsaved version with the saved version on disk.

Is this possible in VSCode natively or else via an extension?

In VIM, such a possibility exists, and here are some references for that:

[Can I see changes before I save my file in Vim?](https://stackoverflow.com/questions/749297/can-i-see-changes-before-i-save-my-file-in-vim/749320#749320)

[VIM - Show the diff of last saved version and current unsaved version](https://stackoverflow.com/questions/760666/vim-show-the-diff-of-last-saved-version-and-current-unsaved-version)

Answer: 
On Windows and Linux, this feature appears in 1.60.0 by default as:

```
{
  "key": "ctrl+k d",
  "command": "workbench.files.action.compareWithSaved"
}

```
Available natively in multiple different ways:

* "Files: Compare Active File with Saved" command from the standard VS Code Command Palette (Ctrl+Shift+P).
* File context menu "Compare with Saved" within the 'OPEN EDITORS' view of the Explorer.
* `Ctrl+K D` keyboard shortcut, as mentioned by Tryer.

The functionality was introduced in v1.15: <https://code.visualstudio.com/updates/v1_15#_compare-dirty-file-with-version-on-disk>