* Replace element.offet* by getBoundingClientRect()

* Use variable to store BoundingClientRect

* Fix cc issue...

* Fix getSelectorFromElement when # is a selector

* Thanks to @vanduynslagerp remove regex to validate selector

- Create backdrop only if the menu is actually open (do not create it if the show event is prevented)
- Drop the backdrop only when the corresponding menu is closed (do not remove if there is no menu to close or if the hide event is prevented)

* Allow to use Tab.js with list-group

* Allow to use list-group with div parent instead of an ul parent

Closes https://github.com/twbs/bootstrap/issues/21635

* Close dropdown menu when focusing an outside element
* Update unit test to new markup

* Remove IE compatibility mode meta tag from docs, examples, and JS tests as we no longer support IE9 and IE8
* update and remove some IE bits from our supported browser page
* update introduction.md to match
* reword starter template intro