[Accessibility] MatDialog is trapping tab key but not headings navigation shortcuts

[opozo]

Bug, feature request, or proposal:

MatDialog is trapping tab key but not headings navigation shortcuts, this is actually working well on AngularJS Material [1] [2]

[1] https://material.angularjs.org/latest/demo/dialog
[2] https://github.com/angular/material/blob/master/src/components/dialog/dialog.js#L1134

What is the expected behavior?

Screen reader should trap tab key as well as headings navigation shortcuts when modal dialog is opened.

What is the current behavior?

Screen reader is not trapping headings navigation shortcuts when modal dialog is opened.

Which versions of Angular, Material, OS, TypeScript, browsers are affected?

"@angular/material": "2.0.0-beta.12"

[jelbourn]

@crisbeto escalating this since it came up from a team in Google. AFAIK the only way to do this would be to add aria-hidden to all of the non-dialog content while its open. I think we should be able to mark all of the siblings to the overlay container as aria-hidden (except for our own aria-live element and anything already aria-hidden), keeping track so we can restore them upon close.

We might also want to deal with stacked dialogs, but that's not as much as a priority.