[terra-application-v3] Evaluate the visibility of focussed scrollable containers in dialog/modal content

[tbiethman]

Story

Description

As a developer of terra-application, I want to determine how best to present focus on scrollable elements in our modal/dialog components

Motivation

So that we meet the expected level of accessibility for those components.

Definition of Done

We'll know we're done when we have determined a design that maximizes the accessibility of those components.

Details

It is not allowed to have two tab stops in a row that do not have a visual indication of focus. In our NotificationDialog, we currently place focus on the dialog container as part of the initial presentation; this does not have a visible focus indicator. The very next tab stop is on the scrollable body of the dialog; this also does not have a visible focus indicator. Thus, we are failing the previously stated requirement.

The initial focus placement cannot change, as its placement aligns with our desire to give users the maximum amount of context when a dialog is presented. So we must investigate whether to:

1. Show a focus indicator when the dialog is initially focused
2. Show a focus indicator when the scrollable body has focus
3. Remove interactions on the body (by making it not scrollable)

This is less of an issue with our expected modal designs, as those should always have a visibly focusable element (a close button, at minimum) between the outer container and its inner scrollable content. But we'll want to keep it in mind for consistency's sake.