Dev: Fix issue with Firefox from the last commit.
Dev: Fix issue with Firefox from the last commit.

- The height of the row read from the DOM isn't actually the rendered
  height - it can be out by <1px, but that's enough to cause a
  cumulative error with the FixedColumn eventually going out of
  alignment. The fix is to set the value to the row that it was read
  from!

markvantilburg/FixedHeader,modulexcite/FixedHeader,KedneckInc/FixedHeader,nickzhuyb/FixedHeader,markvantilburg/FixedHeader,KedneckInc/FixedHeader,joe-meyer/FixedHeader,joe-meyer/FixedHeader,modulexcite/FixedHeader,nickzhuyb/FixedHeader