Use TextBuffer for layouter in TextEdit instead of &str

[kernelkind]

This change allows layouter to use the TextBuffer instead of &str in the closure. It is necessary when layout decisions depend on more than just the raw string content, such as metadata stored in the concrete type implementing TextBuffer.

In our use case, we needed this to support mention highlighting when a user selects a mention. Since mentions can contain spaces, determining mention boundaries from the &str alone is impossible. Instead, we use the TextBuffer implementation to retrieve the correct bounds.

See the video below for a demonstration:

Screen.Recording.2025-02-11.at.4.30.13.PM.mov

Breaking change

This PR introduces a breaking change to the layouter function in TextEdit.

Previous API:

pub fn layouter(mut self, layouter: &'t mut dyn FnMut(&Ui, &str, f32) -> Arc<Galley>) -> Self

New API:

pub fn layouter(mut self, layouter: &'t mut dyn FnMut(&Ui, &dyn TextBuffer, f32) -> Arc<Galley>) -> Self

Impact on Existing Code

• Any existing usage of layouter will no longer compile.
• Callers must update their closures to use &dyn TextBuffer instead of &str.

Migration Guide

Before:

let mut layouter = |ui: &Ui, text: &str, wrap_width: f32| {
    let layout_job = my_highlighter(text);
    layout_job.wrap.max_width = wrap_width;
    ui.fonts(|f| f.layout_job(layout_job))
};

After:

let mut layouter = |ui: &Ui, text: &dyn TextBuffer, wrap_width: f32| {
    let layout_job = my_highlighter(text.as_str());
    layout_job.wrap.max_width = wrap_width;
    ui.fonts(|f| f.layout_job(layout_job))
};

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