Call resize handlers only if chart is displayed

[upphiminn]

In the current implementation the resize handlers are called even if the chart is not displayed. This is not ideal when having one chart in focus and some hidden ones, as all event handlers get called and performance degrades. Also, while Chrome behaves correctly and does not actually change the width of the hidden ones, Firefox tries to resize the hidden elements and sets incorrect full widths, visible after the hidden charts are re-displayed. By checking the offsetParent we can tell if the chart is displayed or not and we can skip resize handling if hidden.

@kt3k

[codecov-io]

Codecov Report

Merging #2164 into master will decrease coverage by 0.09%.
The diff coverage is 20%.

@@            Coverage Diff            @@
##           master    #2164     +/-   ##
=========================================
- Coverage   73.98%   73.88%   -0.1%     
=========================================
  Files          51       51             
  Lines        4178     4185      +7     
=========================================
+ Hits         3091     3092      +1     
- Misses       1087     1093      +6

Impacted Files	Coverage Δ
src/core.js	89.19% <20%> (-0.93%)	⬇️

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[kt3k]

Thanks for the contribution. This change makes sense to me.

offsetParent seems to be null as well when the element itself has position: fixed style, but I think nobody use position: fixed directly to the chart's element. (possibly somebody would use position: fixed for the parent of the chart, but that doesn't cause a problem.)