Addon Docs: iframe size isn't 100% width

[jonhuteau]

Describe the bug

When you navigate to Docs tab, all stories are shown in an iframe with ~320px large instead of 100% width

To Reproduce

https://next--storybookjs.netlify.app/vue-kitchen-sink/?path=/docs/app--app

Expected behavior

iframe should use all available space

Screenshots

System:

Environment Info:

  System:
    OS: Linux 4.15 Ubuntu 18.04.4 LTS (Bionic Beaver)
    CPU: (8) x64 Intel(R) Core(TM) i7-8650U CPU @ 1.90GHz
  Binaries:
    Node: 12.16.3 - /usr/bin/node
    npm: 6.14.5 - /usr/bin/npm
  Browsers:
    Chrome: 83.0.4103.61
    Firefox: 76.0.1
  npmPackages:
    @storybook/addon-a11y: ^6.0.0-beta.12 => 6.0.0-beta.12 
    @storybook/addon-docs: ^6.0.0-beta.12 => 6.0.0-beta.12 
    @storybook/addon-jest: ^6.0.0-beta.12 => 6.0.0-beta.12 
    @storybook/addon-knobs: ^6.0.0-beta.12 => 6.0.0-beta.12 
    @storybook/addon-viewport: ^6.0.0-beta.12 => 6.0.0-beta.12 
    @storybook/addons: ^6.0.0-beta.12 => 6.0.0-beta.12 
    @storybook/vue: ^6.0.0-beta.12 => 6.0.0-beta.12 

Additional context

Upgraded from 6.0.0-beta.6 to 6.0.0-beta.12, don't know which release might be in cause

[shilman]

@ndelangen seems related to #10801, can you take a look?

[ndelangen]

#10877

[shilman]

¡Ay Caramba!! I just released https://github.com/storybookjs/storybook/releases/tag/v6.0.0-beta.14 containing PR #10877 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.