metamask-extension/docs/ui-mock-mode.md

Developing on UI with Mocked Background Process

You can run npm run mock and your browser should open a live-reloading demo version of the plugin UI, just like the npm run ui, except that it tries to actually perform all normal operations.

It does not yet connect to a real blockchain (this could be a good test feature later, connecting to a test blockchain), so only local operations work.

You can reset the mock ui at any time with the Reset button at the top of the screen.