fix(dropdown): z-index set when appended to body

[fbasso]

fix #3061

[codecov-io]

Codecov Report

Merging #3157 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #3157      +/-   ##
==========================================
+ Coverage   92.34%   92.34%   +<.01%     
==========================================
  Files          91       91              
  Lines        3003     3004       +1     
  Branches      483      483              
==========================================
+ Hits         2773     2774       +1     
  Misses        166      166              
  Partials       64       64

Flag	Coverage Δ
#e2e	61.81% <100%> (+0.01%)	⬆️
#unit	89.96% <100%> (ø)	⬆️

Impacted Files	Coverage Δ
src/dropdown/dropdown.ts	94.19% <100%> (+0.03%)	⬆️

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[maxokorokov]

LGTM to me as a fix

But we should probably:

• add e2e tests to open dropdown, tooltip, typeahead, etc. in a modal when attached to body
• put this in scss later as not possible now