fix(positioning): put the popup in a separate layer

[maxokorokov]

Without forcing the popup in a separate layer, Safari and Firefox are doing a lot more work than necessary.

Example: trying to reposition datepicker 3 times, before finding an optimal position.

Quite significant changes in Safari → removes full page layout and compositing.

Safari Before:

Safari After:

In Firefox it removes unnecessary paints as well.

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Concerning the The translate3d/gpu acceleration render a blurry text on chrome, the next line is commented until a browser fix → we're not using translate3d, but translate. And it looks fine in Chrome.

Related to #3199

[codecov-io]

Codecov Report

Merging #3206 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #3206      +/-   ##
==========================================
+ Coverage   92.03%   92.04%   +<.01%     
==========================================
  Files          91       91              
  Lines        3052     3053       +1     
  Branches      504      504              
==========================================
+ Hits         2809     2810       +1     
  Misses        179      179              
  Partials       64       64

Flag	Coverage Δ
#e2e	63.11% <100%> (+0.01%)	⬆️
#unit	89.47% <100%> (ø)	⬆️

Impacted Files	Coverage Δ
src/util/positioning.ts	96% <100%> (+0.03%)	⬆️

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[fbasso]

Unfortunately, the text is blurry on Chrome/Windows with will-change activated:

[maxokorokov]

Ok, so we've figured out that it happens only on Win10 with some font zoom levels in Chrome. Same issue in Bootstrap itself, so will merge.