fix: masonry scroll handler should rely on event.currentTarget issue#1414

[chinmayhem]

#1414

The scroll handler should rely only on the scrolltop of event.currentTarget as any of the descendant elements' scroll event will propagate up to the container element. Relying on event.target seems incorrect.

Added a test case which fails if event.target.scrolltop is used.

Also, updated the function simulateScroll in spec file. As passing currentTarget to Simulate is not supported ( facebook/react#4950 ), changed the simulateScroll function to set the scrollTop on the desired element.

[codecov-io]

Codecov Report

Merging #1420 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master    #1420   +/-   ##
=======================================
  Coverage   90.35%   90.35%           
=======================================
  Files          59       59           
  Lines        1742     1742           
=======================================
  Hits         1574     1574           
  Misses        168      168

Impacted Files	Coverage Δ
source/Masonry/Masonry.js	95.45% <100%> (ø)	⬆️

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[wuweiweiwu]

nice catch! so it will be the element that the scroll event occurs on