`TextInput` internally stores a rope of text content, one segment for
each line. This change separates out the rope into a separate `Rope`
data structure that can be shared with other parts of Servo.
`TextInput` needs to move through text content in a variety of ways,
depending on the keys pressed when interacting with a text area. This
change provides a unified movement API in both `Rope` and in
`TextInput` ensuring that selection is handled consistently (apart from
a few minor quirks [^1]). This simplifies the code an improves
interactive behavior.
[^1]: One of these quirks is that the edit point will move to
directional end of the motion when collapsing a selection, but only when
moving by grapheme or word (and not by line). These quirks existed in an
undocumented way previously and they are preserved with code comments.
Testing: This is covered by existing unit tests (updated for the new
API) and
the WPT suite.
Signed-off-by: Martin Robinson <mrobinson@igalia.com>
2efa7d5d96