It was possible for <input> and <textarea> to incorrectly report the IsReadOnly
property as "true" to UIA, particularly when empty. There is no text leaf in
empty text inputs, so our TextLeafPoint would point to the text field itself.
Code that retrieves attributes for UIA didn't handle this case, which caused AT
issues, such as Narrator failing to switch out of scan mode automatically.
This revision fixes the issue by properly checking for text fields when figuring
out whether a TextLeafPoint's Accessible is read-only. It also adds a test to
verify the same.
Differential Revision: https://phabricator.services.mozilla.com/D237644
739b849487