Issue with deepEqual in SourceContainer

[azaxarov]

Describe the bug
My current Storybook setup works up to and including v6.0.0-beta.37. However, once Storybook dependencies are updated to v6.0.0-beta.38, the stories break on the deepEqual check in SourceContainer. The changes were introduced in #11332.

storybook/addons/docs/src/blocks/SourceContainer.tsx

Line 35 in b25b434

if (!deepEqual(sources, sourcesRef.current)) {

I suspect that it results in a false positive by checking equality of initial sources ({}) with sourcesRef.current that is undefined in my case. An additional check for undefined ref value seems reasonable:

if (sourcesRef.current && !deepEqual(sources, sourcesRef.current))

I appreciate the work you guys are doing and hope this is a relatively easy fix!

To Reproduce
Steps to reproduce the behavior:

1. Go to '...'
2. Click on '....'
3. Scroll down to '....'
4. See error

Expected behavior
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Code snippets
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System:

Environment Info:

  System:
    OS: macOS High Sierra 10.13.6
    CPU: (8) x64 Intel(R) Core(TM) i7-2635QM CPU @ 2.00GHz
  Binaries:
    Node: 12.16.1 - ~/.nvm/versions/node/v12.16.1/bin/node
    npm: 6.14.5 - ~/.nvm/versions/node/v12.16.1/bin/npm
  Browsers:
    Chrome: 83.0.4103.116
    Firefox: 76.0.1
    Safari: 13.0.5
  npmPackages:
    @storybook/addon-a11y: ^6.0.0-beta.40 => 6.0.0-beta.40 
    @storybook/addon-essentials: ^6.0.0-beta.40 => 6.0.0-beta.40 
    @storybook/addon-links: ^6.0.0-beta.40 => 6.0.0-beta.40 
    @storybook/addons: ^6.0.0-beta.40 => 6.0.0-beta.40 
    @storybook/react: ^6.0.0-beta.40 => 6.0.0-beta.40 
    @storybook/theming: ^6.0.0-beta.40 => 6.0.0-beta.40

Additional context
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[shilman]

Yo-ho-ho!! I just released https://github.com/storybookjs/storybook/releases/tag/v6.0.0-beta.41 containing PR #11414 that references this issue. Upgrade today to try it out!

You can find this prerelease on the @next NPM tag.

Closing this issue. Please re-open if you think there's still more to do.

[azaxarov]

Great news! I'll give it a go today.