Add Indeterminate Check Control

[jpante]

🚀 Feature Proposal

Create a new component for indeterminate checkbox controls.

Motivation

Adding indeterminate functionality to checkbox inputs are possible but have limitations which mean it cannot be made completely accessible and usable across all browsers and assistive tech.

We need a separate check control to allow for indeterminate states that is separate to the check input.

Some Considerations

• This means the indeterminate check control would not support things like auto population of form inputs.

Example

https://baseweb.design/blog/screen-reader-improvements#components-should-announce-their-state

[NicholasBoll]

This doesn't have to be slated for v6, it could be post release of v5 if it is a new component. I think we introduce a new component, deprecate indeterminate of the current Checkbox and remove that prop in v6.

[jaclynjessup]

When syncing with A11Y we asked several quesitons to gain clarity for this request: Was it not accessible because of IE11? Not specific to IE11 Is a redesign needed? No, this is a code issue. aria-checked="mixed" attribute isn't consistently supported, a work around for it exists

Additional guidance will be needed in the dev docs (on aria labels) + updated usage guidance on when to use an indeterminate checkbox, when to consider something else