fix popups being covered by page scrollbar

[nightwing]

Fixes the issue where autocomplete popup and doc tooltip could be covered by page scrollbar, or could extend outside of viewport.

[codecov]

Codecov Report

Attention: Patch coverage is 62.22222% with 17 lines in your changes missing coverage. Please review.

Project coverage is 87.09%. Comparing base (cd78a94) to head (e55d8bd).

Files with missing lines	Patch %	Lines
src/autocomplete.js	55.26%	17 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5763      +/-   ##
==========================================
- Coverage   87.10%   87.09%   -0.02%     
==========================================
  Files         602      602              
  Lines       44092    44114      +22     
  Branches     7234     7240       +6     
==========================================
+ Hits        38407    38420      +13     
- Misses       5685     5694       +9     

Flag	Coverage Δ
unittests	87.09% <62.22%> (-0.02%)	⬇️

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