add "will-change" property

[marcobiedermann]

Using postion: fixed on an element causes a lot of repaint on scrol. Adding will-change: transform will promote the element to it's own layer which improves the framerate.

Article by @paullewis about will-change

[cvrebert]

This would probably be more appropriate for Bootstrap v4 (the v4-dev branch) than Bootstrap v3. I'm doubtful we'd want to take the potential stability risk, given that v3 is in maintenance mode.

Also, this sounds morally equivalent to the translateZ hack, which we tried before but had to revert due to browser bugs and due to new stacking contexts being unintuitive in some cases. See #13649

[marcobiedermann]

@cvrebert Yes, you are right. But this is the "right" way to promote an element to it's own layer. Adding translateZ is more of a hack

[cvrebert]

Closing for v3 due to aforementioned stability concerns.
You can try proposing this change for v4 though.