For paragraphs, divs, spans, etc., a11y focus on Android goes to text leaf Accessibles, rather than to the HyperTextAccessible container.
This does make sense, as these containers frequently embed other content, so the text needs to be reachable as a separate item.
However, previously, performing text navigation on these text leaf Accessibles returned the HyperTextAccessible parent.
This isn't supported by Talkback, and even if it were, it causes other problems; e.g. a11y focus being lost if the user was focused on a child other than the first child of such a container.
Therefore, if text navigation was performed on a text leaf Accessible, we now return a result within the text leaf Accessible if possible, rather than the HyperTextAccessible.
1. Make AccessibleWrap::GetTextContents support text leaf Accessibles (for both local and remote proxied Accessibles).
This is used when providing text for text traversal events.
2. When navigating text on Android, we use Pivot::Next/PrevText.
However, this will always return a HyperTextAccessible, even when starting on a text leaf.
Therefore, if the result from Pivot::Next/prevText resides entirely within the same text leaf, translate the offsets from the HyperTextAccessible so they're relative to the text leaf and return the text leaf.
3. Pivot::Next/PrevText already supported starting from a text leaf Accessible.
However, they ignored the offsets, which meant that navigating from a text leaf would always navigate to the start/end of the text leaf.
Now, if a text leaf is passed to Pivot::Next/PrevText, the offsets (if specified) are translated to the HyperTextAccessible parent first.
4. Adjust the existing character/word/line tests so they ensure that navigation returns the node that has a11y focus; i.e. the text leaf.
Differential Revision: https://phabricator.services.mozilla.com/D57269
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