FAST_REFRESH behaviour

[hyperknot]

1. FAST_REFRESH is mentioned as experimental / unstable in the original release log
   here: Add experimental react-refresh support #8582 and here:
   

   create-react-app/packages/react-scripts/config/env.js

   Lines 97 to 98 in 3f5dea9

   	// react-refresh is not 100% stable at this time,
   	// which is why it's disabled by default.

2. I can confirm it's very unstable / unusable today. It doesn't detect any change in non-UI components, for example MobX stores, thus the whole refresh is broken, I always have to reload manually now.

3. Against what is written in the release log and source code comments, it is actually ENABLED by default today.
   

   create-react-app/packages/react-scripts/config/env.js

   Line 100 in 3f5dea9

   FAST_REFRESH: process.env.FAST_REFRESH !== 'false',

I believe this line need to be changed to === 'true', to make it disabled by default. Also, a small explanation should be added to the next release log, this feature is really not documented at all.

[ottoo]

I noticed an issue with the fast refresh and Apollo local state today as well. If I have fast refresh enabled, the very first value I get from the cache is undefined even though the value exists when I check Apollo devtools and local storage. We're using a boolean in the Apollo local state to denote that the user is logged in so when it's undefined the very first time, the user is logged out.

[n8sabes]

Initially, I thought React Hot Loader was broken with lazy loaded and async imported modules. However, upon further investigation, I agree with @hyperknot that Fast Refresh is enabled by default.

Now, live edits ALWAYS FAIL TO REFRESH. While I have high hopes for Fast Refresh, the OPT-OUT workaround is a must:

.env

FAST_REFRESH=false

Related CRA issues, include: #9904, #10078, #10539, etc.