Previously, TextInputModel's SetEditingState method was a 1:1 mapping of
the underlying protocol used on the text input channel between the
framework and the engine. This breaks it up into two methods, which
allows the selection to be updated independently of the text, and avoids
tying the API the the underlying protocol.
This will become more important when we add additional state to support
composing regions for multi-step input methods such as those used for
Japanese.
SetText resets the selection rather than making a best-efforts attempt
to preserve it. This choice was primarily to keep the code simple and
make the API easier to reason about. An alternative would have been to
make a best-effort attempt to preserve the selection, potentially
clamping one or both to the end of the new string. In all cases where an
embedder resets the string, it is expected that they also have the
selection, so can call SetSelection with an updated selection if needed.
The Windows, Linux, and GLFW embeddings (which all share a common code
ancestry) pass TextInput.setEditingState selection base and extents
straight through to the shared text model class. The model expects those
values to be valid, but the framework sends -1/-1 for "invalid"
selections, which happen for some empty text cases (e.g.,
TextFieldController.clear()).
This translates those invalid selection values to an empty selection at
the start of the string, as expected by the model.
Fixes https://github.com/flutter/flutter/issues/59140
