Markdown editor toolbar buttons defaults to tabIndex=-1

[tverney]

Is your feature request related to a problem? Please describe.

Due an accessibility report, we've saw that the current version doesn't support change tabindex="-1" to "0". Currently behaviour removes removes the element from the navigation sequence.

Describe the solution you'd like

Wondering if would be possible to expose the tabindex attribute programmatically. Also want to understand the reasons to fix that in the code.

Describe alternatives you've considered

We can make it manually focusable using javascript, but would be easier just setting the tabindex. Thanks in advance for the review.

Reference in code:

 el.tabIndex = -1;