Editor content width is the max child width

[amantoux]

When used with TextWidthBasis.longestLine, the editor width would accumulate child width instead of using the children max width.

Fixes #558

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.30%. Comparing base (b4c4d7a) to head (3db1148).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #557   +/-   ##
=======================================
  Coverage   89.30%   89.30%           
=======================================
  Files          63       63           
  Lines       10985    10985           
=======================================
  Hits         9810     9810           
  Misses       1175     1175           

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[Amir-P]

LGTM!