Calculate column widths using floating point width for parent container

[craiggoldstone]

I encountered a problem where columnWidth * columnCount was greater than the width of the parent container. The root cause is because column calculation was done using the Integer width of the container (.offsetWidth) but should be done using the Float width of the container (.getBoundingClientRect().width), since the float width might be fractionally larger than the rounded .offsetWidth()

[craiggoldstone]

Closed to make individual PRs