fix: custom scrollbar breaks csp mode

[nightwing]

fixes #4966

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[codecov]

Codecov Report

Base: 86.06% // Head: 86.06% // No change to project coverage 👍

Coverage data is based on head (c86d548) compared to base (96e4066).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #4967   +/-   ##
=======================================
  Coverage   86.06%   86.06%           
=======================================
  Files         544      544           
  Lines       41432    41432           
  Branches     6537     6537           
=======================================
  Hits        35659    35659           
  Misses       5773     5773           

Flag	Coverage Δ
unittests	86.06% <100.00%> (ø)

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Impacted Files	Coverage Δ
src/scrollbar_custom.js	95.15% <100.00%> (ø)

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