* Throw error when a plugin is in transition
* Add unit tests about plugins in transition

When the rubberband effect causes Safari to scroll past the top of the
page, the value of scrollTop becomes negative. If the offset of the first
ScrollSpy target is 0 - essentially if the target is at the top of the
page - then ScrollSpy should not clear the active item. Conceptually, the
first item should remain active when rubberbanding past the top of the
page.

This commit fixes issue #21055 by verifying the first scrollspy target is
not at the top of the page before clearing the active nav-item.

See https://travis-ci.org/twbs/bootstrap/jobs/179010884 for build failure

Dropdown perf - on keyboard nav, find active item in the active dropdown rather than the whole document (#19953)

* Fix bug with dropdown tab links not deactivating when other tab or dropdown link is clicked

* Revise bug fix for more stability

* Explore responsive display utils, but with a twist: lowest breakpoint has no breakpoint modifier in the class name
* make floats use the same format, add float-none mixin

* Fix #17964

Some browsers are lazy when updating dom elements after transition effects. This can be fixed by reading element properties such as offsetHeight or offsetWidth. However, creating a function using the Function constructor just to access such element, results in a violation of Content Security Policy (where applied), which in turn crashes the application. This fix actually reverts to the way this was handled in v3 and should work as intended.