only skip animation if !isTabVisible, not the whole rendering

[panthony]

It fixes a regression introduced by f4a163f and closes #2533

[codecov-io]

Codecov Report

Merging #2671 into master will decrease coverage by <.01%.
The diff coverage is 95.23%.

@@            Coverage Diff             @@
##           master    #2671      +/-   ##
==========================================
- Coverage   80.13%   80.12%   -0.01%     
==========================================
  Files          57       57              
  Lines        4520     4519       -1     
==========================================
- Hits         3622     3621       -1     
  Misses        898      898

Impacted Files	Coverage Δ
src/core.js	90.11% <95.23%> (-0.02%)	⬇️

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[panthony]

Hum... I just saw this commit: 19a54ec but it did not fixed the issue for me.

The pies are here, the line's dot are here, but not the bars, not the lines.

if I click back on the tab, the focus event is not sent but document.hidden is false. If I click anywhere in the window when the focus event is sent.

I'm using Chrome 75.

Moreover, all my charts (and I mean several dozens) are now redrawn just because I switched tab which is annoying, I think i'll have to revert that part for me.

[kt3k]

@panthony Thanks! LGTM