Errors in Modal._setScrollbar()

[cvrebert]

See #18366 (comment)

[deilv]

@cvrebert you can mark this as confirmed, there is indeed a bug in Modal._setScrollbar.

Line 430 should be:

`${this._originalBodyPadding + this._scrollbarWidth}px`

Already tested this and the fix works as intended, however, the fixed navbar still scrolls to the left. I can only assume that the first part of the function was about to fix the navbar padding, but then something went wrong along the way. I'm still working on it.

[deilv]

I believe I have a rather complete fix for both the body and fixed-navbar scrolling related to the modal. Please check this codepen to see if it works well for you (already tested on FF, Edge and IE11). PR coming soon.

http://codepen.io/deilv/pen/pjMNXg

Note: I rewrote both _setScrollbar and _resetScrollbar, got rid of a variable in the proccess (_originalBodyPadding).

[deilv]

Made further changes to the code to address all possible scenarios I could think of. Now it supports pre-existing % based padding and also adjusts/restores all fixed content on the page.

You can check the updated version of the codepen posted above. I'll run some more tests to see all is ok before I send a pull request.