Fix doubled scroll issue when iframe content is higher than frame height

[bbronisz]

Q	A
Bug fix?	[ x]
New feature?	[ ]
New sample?	[ ]
Related issues?	fixes #X, partially #Y, mentioned in #Z

What's in this Pull Request?

It fixes double scrolls on different browsers due to wrong height calculations of iframe content, due to:

• selectors without . for classes
• marginTop style's value used to calculate marginBottom
• usage of style property on element (it was empty e.g. on Chrome) instead of window.getComputedStyle
• too small value used for padding (at least on Firefox and Chrome)

[codecov-io]

Codecov Report

Merging #431 into dev will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##              dev     #431   +/-   ##
=======================================
  Coverage   73.31%   73.31%           
=======================================
  Files          18       18           
  Lines         802      802           
  Branches      154      164   +10     
=======================================
  Hits          588      588           
  Misses        164      164           
  Partials       50       50

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 121af1a...d58260d. Read the comment docs.

[AJIXuMuK]

Great job @bbronisz!
Thank you for the fixes!