Closed expansion panel will focus hidden elements

[csvn]

Bug, feature request, or proposal:

Bug

What is the expected behavior?

If an md-expansion-panel has focus and is closed, the next item to gain focus when pressing tab should be the next md-expanion-panel.

What is the current behavior?

If a closed and focused md-expansion-panel has any buttons (or similar focusable targets), tab will go to those elements. This occurs even though the elements are not visible on the page.

What are the steps to reproduce?

https://github.com/angular/material2/blob/master/src/demo-app/expansion/expansion-demo.html

The expansion panel demo should be enough to reproduce:

1. Focus the closed "Single expansion panel"
2. Press tab
3. Focus will land on the cancel button

What is the use-case or motivation for changing an existing behavior?

Usability and expected behavior.

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

Angular Material v2.0.0-beta.7.

Is there anything else we should know?

I don't think so, but tell me if I missed something.

[angular-automatic-lock-bot]

This issue has been automatically locked due to inactivity.
Please file a new issue if you are encountering a similar or related problem.

Read more about our automatic conversation locking policy.

_{This action has been performed automatically by a bot.}