Don't scroll canvas when dialog is open

[ericnewcomer]

Fixes: #746

[codecov]

Codecov Report

Merging #754 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #754      +/-   ##
==========================================
+ Coverage    64.2%   64.21%   +<.01%     
==========================================
  Files         164      164              
  Lines        6074     6075       +1     
  Branches     1410     1411       +1     
==========================================
+ Hits         3900     3901       +1     
  Misses       1745     1745              
  Partials      429      429

Impacted Files	Coverage Δ
src/components/flow/Flow.tsx	66.66% <100%> (+0.35%)	⬆️

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