Consideration of keyup event for Polyfill instead of keydown

[joe-watkins]

In the polyfill, keydown event and mouseup event are used for the spatial navigation.

I'm curious to see if there is a great case for leveraging a keydown event as apposed to a keyup event. Keyboard users who may have mobility/dexterity type disabilities could benefit from an event that they have more control over the timing of it being fired. e.g. someone with tremors using a keyboard.

WCAG 2.1 A 2.5.2 Pointer Cancellation currently speaks to this concept for touch/mouse. (should clearly extend to keyboard events IMO)
https://www.w3.org/WAI/standards-guidelines/wcag/new-in-21/#252-pointer-cancellation-a

Browser manufacturers may be looking to the polyfill for guidance and I was interested in seeing if maybe a keyup event could be the default vs requiring an override.

https://wicg.github.io/spatial-navigation/#handling-browser-events

[jihyerish]

Thanks for raising this issue!

However, this issue is about the spatial navigation polyfill, not the spec.
Issues about the polyfill have been discussed in https://github.com/WICG/spatial-navigation,
so I've migrated this issue to WICG/spatial-navigation#214.