Fix CSS for collapsed cell and remove outputExpanded

[captainsafia]

Resolves #4448

As it turns out, we already had the toggle expanded property in the cell menu but we were missing the CSS for it. This is implemented now.

[codecov]

Codecov Report

Merging #5085 into master will not change coverage.
The diff coverage is 71.42%.

@@           Coverage Diff           @@
##           master    #5085   +/-   ##
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  Coverage   72.44%   72.44%           
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  Files         158      158           
  Lines        4584     4584           
  Branches      684      684           
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  Hits         3321     3321           
  Misses        978      978           
  Partials      285      285           

[CrystallineCat]

Looks good! Left one comment in the code.

[CrystallineCat]

Nit: change the outputExpanded in the name of the test:

test("changes outputExpanded set", () => ...)

[badiku]

packages/styles/themes/base.css
.nteract-cell-outputs {

overflow-y: scroll

doesn't this line you added force scroll bar to appear, even though there's only one line?

I overrided this boring scroll bar like this :

from IPython.display import HTML
style = '''
    <style>
        div.nteract-cell-outputs {overflow-y: auto }
    </style>
    '''
HTML(style)