Add a bottom affixed element to affix visual tests.
Refactor determining affix state into a separate expanded method
  in order to handle multiple edge cases, specifically when the document
  height is dynamic.
Always reposition an affix that is affixed to the bottom.
Fix issue where bottom affixed element floats over the footer when the
  document height is smaller than the viewport height.
cleanup style
Merge pull request #14207 from twbs/charlesbjohnson-affix-bottom-when-dynamic-height

Charlesbjohnson affix bottom when dynamic height