409fb68d25
Fixes https://github.com/flutter/flutter/issues/138371 When a new route pops up the expectation is that the screen reader focuses on something in the new route. Since routes typically result in DOM nodes being replaced, the current effect is that the screen reader simply unfocuses from the page, causing the user to have to refocus on back on the page and look for elements to interact with, which is a poor user experience. The current workaround is to use `autofocus`, but that doesn't scale as it's easy to forget, and if the route in question is maintained by a different person you may not even have enough control over it to set `autofocus` on anything. For example, this is the case with Flutter's default date picker. All you have is `showDatePicker` and there's no way to control the focus. With this change the route (managed by the `Dialog` primary role) will check if a widget requested explicit focus (perhaps using `autofocus`), and if not, looks for the first descendant that a screen reader can focus on, and requests focus on it. The auto-focused element does not have to be literally focusable. For example, plain `Text` nodes do not have input focus (i.e. they are not `isFocusable`) but screen readers can still focus on them. If such an element is found, the web engine requests that the browser move focus to it programmatically (`element.focus()`), which causes the screen reader to move the a11y focus to it as well, but it sets `tabindex=-1` so the element is not focusable via keyboard or mouse.
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