mdDialog: user should not be able to interact outside of dialog

[marcysutton]

So that keyboard and screen reader users cannot get lost while a dialog is open, focus should be limited to controls within the dialog (Cancel button, Accept button, etc.). Modal dialogs should not allow the user to interact with the page behind it.

Related: clicking on the background behind an alert or confirmation dialog should not do anything.

[marcysutton]

This also might require us to remove the parent option for alert/confirm dialogs. Custom dialogs could still use it.

[ThomasBurleson]

@marcysutton - the dialog mask currently traps the outside-dialog clicks right ?

[marcysutton]

@ThomasBurleson by default, yes...clicking the background is the same action as clicking the "cancel" button. But any other links on the page shouldn't be accessible.

I found the parent option is indeed necessary so that people can provide a different wrapper as a dialog target (such as a div with a CSS class on it). I'm doing two things to fix our docs:

1. Pass parent: document.body to our demos to deal with multiple ng-apps and make modals cover the entire screen.
2. Write docs indicating that parent should only be used on nodes that cover the entire screen.

For accessibility, I still have to disable every node that isn't the dialog so it won't be accessible from a screen reader using aria-hidden. I have focus handled already in #1350.

[yarl]

One more thing to add here: there should be no possibility to focus element behind dialog window using [tab] key. So basically, tab focus order should be looped inside dialog.