If you use the scrollbar, a `mousedown` and `pointerdown` event is fired
before you start interacting with the scrollbar. If you use the `scroll`
event, then the callback is fired while scrolling.

To improve performance a bit more, we will set the activation trigger in
the `mousedown` event because we only need to set it once.

If you are done scrolling, we don't reset it (we didn't do that before
either), but instead we rely on other events to override the trigger
(e.g.: you start using the keyboard).

The big benefit is that this now only calls the `setActivationTrigger`
once, instead of `n` times with the same value.

This is a similar performance trick as before. We only need to set the
activationTrigger once, so there is no need to keep calling this
function for no reason.