Browser: Chrome Version 74.0.3729.169 (Official Build) (64-bit)
I was reviewing https://github.com/sourcegraph/sourcegraph/pull/4403/files and whenever I hovered over a symbol I only got a spinner.
Opening the console I see the following error, multiple times:
Uncaught Error: Extension context invalidated.
at Object.send (inject.bundle.js:149)
at MessagePort.t.onmessage (inject.bundle.js:149)
send @ inject.bundle.js:149
t.onmessage @ inject.bundle.js:149
Live footage:

I believe your extension got updated :) This occurs whenever the background page is reloaded, and a new browser extension version got deployed last night. Try a hard reload and let us know if this happens again.
Wow, yeah, hard reloading fixed it. Is there any way to catch this and let the user know?
We could explore listening to the onInstalled event (which is also fired on update), and disconnecting the stale content script + re-injecting when that happens. Some people have gone down that route.
There seems to be a solution: https://stackoverflow.com/a/55336841/288906
Or you can catch that error and suggest a reload
Most helpful comment
I believe your extension got updated :) This occurs whenever the background page is reloaded, and a new browser extension version got deployed last night. Try a hard reload and let us know if this happens again.