docs(core): how and when to use delegate-focus-mixin.js

[lkraav]

https://github.com/web-padawan/aybolit/blob/116c91840792132bece7f784a364f707537d6d9f/packages/core/src/mixins/delegate-focus-mixin.js

Some @aybolit/core components are using it, but a few are not. What is the decision criteria?

lit/lit-element#586 may be related on upstream end?

[web-padawan]

The main reason is that only Chrome currently supports the sequential focus navigation:

this.attachShadow({mode: 'open', delegatesFocus: true});

The mixin is needed whenever you have a focusable element (input, button etc) within a shadow root. https://github.com/vaadin/vaadin-control-state-mixin does pretty much the same thing.

[lkraav]

Maybe a docs(...): ... commit would be a good idea about this in some README.md.

[web-padawan]

The mixin is considered implementation detail at the moment. But yes, we could add some docs.