fix(button-toggle): able to focus disabled button via click

[crisbeto]

Along the same lines as #15499. Fixes users being able to focus a disabled button toggle by clicking on it. The issue comes from us preserving the -1 tabindex, even if the button is disabled.

[jelbourn]

So, funny story, turns out we need to remove the tabindex="-1" from these components where we're redirecting focus to an underlying element. The problem is that some screen-readers (notably ChromeVox) will stop on anything with a tabindex like this when navigating through items (not with tab, but with the reader's own navigation). This leads to situations where the user ends up landing on both <mat-toggle-button> and then the internal <button>, making it seem like they hit the same thing twice.

But button-toggle in particular, I'd like to resolve this by eventually changing to <button mat-toggle-button>, but I'm not sure there's anything we can do for checkbox and radio button.

[crisbeto]

The reason why we started setting a tabindex was to support cases like <mat-checkbox cdkFocusInitial>. If we were to remove the tabindex again, it will break those cases.

[jelbourn]

I think we'll have to use a different approach for that. Probably something like having MatCheckbox provide some symbol like IndirectFocusTarget with the custom action and having the cdkFocusInitial use that

[crisbeto]

I don't think we can do that, because in a lot of cases we attach the focus trap directly to a DOM node which means that can't use a ContentChild to find the focus target.

[jelbourn]

What if we just straight up monkey-patch the focus method on the host element?

[crisbeto]

That technically would work, but it feels a little hacky. Also we have a check on the focus trap that logs a warning if the cdkFocusInitial element isn't focusable which we'd have to remove.

[jelbourn]

Let's discuss this in the team weekly and decide on a final course of action

[crisbeto]

Closing since this is no longer relevant after 39e4e24.

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