Fix preview scrolling

[Hypnosphi]

Issue: #1779

What I did

Used an overlay div instead of pointer-events: none when dragging.
See tomkp/react-split-pane#30, Chromium bug

How to test

1. Open cra-kitchen-sink in Chrome
2. Resize browser window so that it's lower than the preview content
3. Resize some of the panes (stories / addon)
4. Try to scroll the preview

[codecov]

Codecov Report

Merging #1782 into release/3.3 will increase coverage by <.01%.
The diff coverage is 60%.

@@               Coverage Diff               @@
##           release/3.3    #1782      +/-   ##
===============================================
+ Coverage        23.13%   23.13%   +<.01%     
===============================================
  Files              253      253              
  Lines             5745     5748       +3     
  Branches           685      678       -7     
===============================================
+ Hits              1329     1330       +1     
- Misses            3921     3937      +16     
+ Partials           495      481      -14

Impacted Files	Coverage Δ
app/react/src/server/index.html.js	0% <ø> (ø)	⬆️
app/react-native/src/server/index.html.js	0% <ø> (ø)	⬆️
app/vue/src/server/index.html.js	0% <ø> (ø)	⬆️
lib/ui/src/modules/ui/components/layout/index.js	76.27% <60%> (-2.31%)	⬇️
lib/ui/src/modules/ui/containers/stories_panel.js	25.71% <0%> (ø)	⬆️
addons/events/src/components/Event.js	0% <0%> (ø)	⬆️
addons/info/src/components/Props.js	37.2% <0%> (ø)	⬆️
addons/knobs/src/components/types/Select.js	7.81% <0%> (ø)	⬆️
lib/ui/src/modules/shortcuts/actions/shortcuts.js	22.41% <0%> (ø)	⬆️
addons/knobs/src/KnobManager.js	32% <0%> (ø)	⬆️
... and 22 more

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[ndelangen]

Tested