Even when the affix plugin sets the 'affix' class to the affix element, it still has 'top' and 'position' attributes

[jtzikas]

I have been having lots of issues with the affix plugin whith a complicated dynamic structure and I have tracked it down to line 69 in affix.js. https://github.com/twbs/bootstrap/blob/master/js/affix.js#L68

The use case is that the issue appears when the affix element transitions from affix-bottom to affix. In my case the affix plugin did not clean up the 'top' and 'position' attributes and the element was stuck floated at the affix-bottom position.

The reason is that 'this.unpin' was calculated to 0. When the value is 0 the comparison in the bellow if case is evaluated to false and the element is never cleaned up:

if (this.unpin) this.$element.css('top', '')

So I replaced it with with this block:

if (this.unpin != null) {
        this.$element.css('top', '')
        this.$element.css('position', '')
}

[cvrebert]

Probably fixed by #12982. If not, please let us know.