Many tooltips use an animation effect. An example is the tooltips here on github: if you hover over the notification icon in the top-right, an opacity 0->1 animation is used for the tooltip. A simple proposal for anchor positioning would be:

position-visibility-show-animation: [animation-name];
position-visibility-hide-animation: [animation-name];

The current spec for position visibility (link) states that hidden anchors are strongly hidden. This would need to change so that it takes effect after the hide animation, and stop taking effect before the show-animation.

@tabatkins @una