Initial docs page renders twice, leading to first render not being cleaned up properly

[tmeasday]

Describe the bug

1. Create a v7 store repro with the CLI template: e.g. https://github.com/tmeasday/sb-17214
2. Visit the default URL (no story specified)
3. Go to the second story (Button:Primary)
4. Change an arg
5. Notice the initial docs only story is rendered

Explanation of bug

The issue is that with the async story loading the following sequence of events happens:

1. SB loads, no story specified,
2. Preview picks first story and:
   a. emits storySpecified event
   b. starts rendering
3. Rendering is async as the story must be loaded (v7 store) -- renderSelection() function is running.
4. Meanwhile, the manager emits setCurrentStory with the selected story (<- this bit is a bit weird but is existing behaviour in SB).
5. Preview gets message from 4, and starts renderSelection() again
6. Function checks if the story is unchanged, but as 3 hasn't finished yet, this.previousStory is not yet set.
7. So the docs is rendered a second time, setting up handlers twice
8. When you browse away from docs, the first set of handlers are not torn down.

[tmeasday]

@ghengeveld talked about this a bit more with @shilman and realised the issue is actually very similar to issues we've faced with the render cycle. A summary of the issue is: What guarantees do we make if a setCurrentStory is received while the previous render process is still ongoing?

There are a few async steps in the "render process" that are relevant:

1. loading the CSF file (this issue)
2. cleaning up the previous render (can be async if we need to cancel a play function)
3. running loaders
4. rendering the story
5. running the play function.

I believe we thought a bunch about what happens during steps 3-5 but not at all about steps 1-2. I'm wondering your thoughts -- should we just add them as render phases and handle them in a similar way with the abort controller? Or something else?

[shilman]

¡Ay Caramba!! I just released https://github.com/storybookjs/storybook/releases/tag/v6.5.0-alpha.47 containing PR #17598 that references this issue. Upgrade today to the @next NPM tag to try it out!

npx sb upgrade --prerelease

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