Support :focus-visible CSS pseudoclass

[Michael Strauß]

The CSS specification includes some suggestions for the heuristics
used to determine whether or not focus should be indicated:
https://drafts.csswg.org/selectors-4/#focus-visible-pseudo

Here's how I think these suggestions apply to JavaFX:


> If the user has expressed a preference (such as via a system preference or a browser setting) to always see a visible focus indicator, indicate focus regardless of any other factors. (Another option may be for the user agent to show its own focus indicator regardless of author styles.)

As far as I know, no supported OS platform has a system-wide setting
for focus indicators. Absent such a setting, I think app developers
can easily support always-on focus indicators by using the :focused
pseudoclass, instead of :focus-visible.


> If the element which supports keyboard input (such as an input element, or any other element that would triggers a virtual keyboard to be shown on focus if a physical keyboard were not present), indicate focus.

As with the first suggestion, this can be achieved by using the
:focused pseudoclass.


> If the user interacts with the page via keyboard or some other non-pointing device, indicate focus. (This means keyboard usage may change whether this pseudo-class matches even if it doesn’t affect :focus).

This behavior is implemented in the proposed PR.


> If the user interacts with the page via a pointing device (mouse, touchscreen, etc.) and the focused element does not support keyboard input, don’t indicate focus.

This behavior is implemented in the proposed PR, but without regards
to whether or not the focused element supports keyboard input. If app
developers want to have always-on focus indicators for keyboard input
elements, the :focused selector can be used.


> If the previously-focused element indicated focus, and a script causes focus to move elsewhere, the newly focused element should indicate focus.
> Conversely, if the previously-focused element did not indicate focus, and a script causes focus to move elsewhere, the newly focused element should also not indicate focus.

This behavior is difficult to implement in JavaFX, because there is no
way for a node to know whether or not it gained focus
programmatically, or as a result of a control skin invoking
requestFocus(). Supporting this scenario would require adding new
public API and potentially changing lots of skins to account for the
distinction between programmatic and mouse-based focus traversal. I
think the impact on existing code would be too large for a marginal
benefit.