Modal tabbing and shift+tabbing behavior

Created by: andrewnding

Not sure if this is intended behavior, but the outermost div with the modal class has a tabindex of -1, which seems to me that it should get initial focus when the modal opens, but should not be reachable again by tabbing. However you can tab to it, which makes it seem like the focus highlighting has disappeared.

Another related possible issue is that when shift+tabbing, nothing happens if you try to shift tab past the first focusable element (so that the focus is on the outer div with tabindex -1). I would expect the focus to go from the first focusable element to the last focusable element, much like how if you tab forward, it jumps from the last focusable element to the first focusable element.

Modal demo for reference: https://getbootstrap.com/docs/4.0/components/modal/#live-demo