VS Code seems to maintain it's own record of working changes on a file. (See the "dot" next to the filename in the image, which indicates unsaved changes.)
I want it to completely forget these changes and load the file from disk.
Closing and re-opening the file and/or entire project doesn't do it. Where is the button or what is the command to just discard changes?
This feature where VS Code remembers unsaved changes when you close the workspace is called "Hot Exit". You can find docs about it here: https://code.visualstudio.com/docs/editor/codebasics#_hot-exit.
If you want VS Code to not keep track of your unsaved changes, then put "files.hotExit": "off"
in your settings.json file. That will cause it to prompt you on whether you want to save unsaved files before exiting, at which point you can select the "Don't Save" option.
If you've already opened a file where it recalled the unsaved changes it remembered, just close that editor tab, and select the "Don't Save" option in the prompt that appears about unsaved changes. Then reopen that file.