Fix Horizontal Scrollbar of DownPanel

[tobilen]

Issue: #2387

What I did

Extend flex property to "1 1 auto", which sets a flex-basis of auto instead of 0%. While this wont work in IE10 or below, those are not supported by storybook anyway :)

How to test

• start locally
• with firefox (v57 and above), navigate to http://localhost:9010/?selectedKind=Addon%20Info.React%20Docgen&selectedStory=Comments%20from%20PropType%20declarations&full=0&down=1&left=1&panelRight=1&downPanel=storybook-addon-background%2Fbackground-panel
• try scrolling horizontally in the right panel

Is this testable with jest or storyshots?
no, since styles with css modules are not snapshot.

Does this need a new example in the kitchen sink apps?
no, there is already one that shows the issue (see link above)

Does this need an update to the documentation?
no

[codecov]

Codecov Report

Merging #2410 into release/3.3 will not change coverage.
The diff coverage is n/a.

@@             Coverage Diff              @@
##           release/3.3    #2410   +/-   ##
============================================
  Coverage        18.76%   18.76%           
============================================
  Files              348      348           
  Lines             8200     8200           
  Branches           907      912    +5     
============================================
  Hits              1539     1539           
+ Misses            5986     5959   -27     
- Partials           675      702   +27

Impacted Files	Coverage Δ
.../ui/src/modules/ui/components/addon_panel/style.js	0% <ø> (ø)	⬆️
addons/jest/src/components/Indicator.js	0% <0%> (ø)	⬆️
lib/ui/src/modules/ui/components/layout/index.js	29.28% <0%> (ø)	⬆️
...codemod/src/transforms/update-organisation-name.js	40.62% <0%> (ø)	⬆️
addons/events/src/components/Event.js	0% <0%> (ø)	⬆️
...-native/src/preview/components/OnDeviceUI/index.js	0% <0%> (ø)	⬆️
lib/codemod/src/transforms/update-addon-info.js	50% <0%> (ø)	⬆️
lib/ui/src/modules/ui/components/search_box.js	35.95% <0%> (ø)	⬆️
lib/ui/src/modules/ui/containers/search_box.js	23.52% <0%> (ø)	⬆️
addons/knobs/src/components/types/Date/index.js	23.52% <0%> (ø)	⬆️
... and 49 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 295c8b4...6f22aab. Read the comment docs.

[tobilen]

im not really sure how to convince codecov that the coverage didnt change. help? 🐼

[Hypnosphi]

Live example for testing: https://deploy-preview-2410--storybooks-cra.netlify.com/?selectedKind=Addon%20Info.React%20Docgen&selectedStory=Comments%20from%20PropType%20declarations&full=0&addons=1&stories=1&panelRight=1&addonPanel=storybook-addon-background%2Fbackground-panel

[Hypnosphi]

LGTM, tested in FF 57

[Hypnosphi]

@tobilen would you mind resolving the conflicts?

[tobilen]

@Hypnosphi done