In the bug ( crbug.com/703816 ) we found that getComputedStyle().top returns wrong value for sticky positioned elements.

According to flackr@:

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It's worth noting the computed style behavior is pretty inconsistent across browsers today:

Computed style of top: auto (test page http://output.jsbin.com/wunoxod/quiet) :

position:   static  relative  sticky   absolute  fixed
Chrome 58   auto    0px       0px      224px     296px
Firefox 53  auto    0px       auto     224px     296px
Edge 14     auto    auto      n/a      auto      auto
Safari 10   auto    auto      auto     auto      auto

For position relative, returning a length or auto has very little difference. For every other position type though, if you were to set the returned length value it would have implications to the layout position. For fixed if you used the bottom value it would now track the viewport size, for absolute it would no longer move with layout, and with sticky it starts moving with scroll.

(https://bugs.chromium.org/p/chromium/issues/detail?id=703816#c13)
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The sticky case is about to work correctly but fixed positioned elements still have interop issues.