Add Style::strong_font/emphasis_font for bold/italic rendering

[w4nderlust]

Summary

This adds two optional fields to Style that let users swap in real bold and italic font families for RichText::strong() and RichText::italics(), instead of relying on the default approximations (color change for bold, vertex skew for italic).

Currently, RichText::strong() only changes the text color (via Visuals::strong_text_color()), which does not produce visually bold text. And RichText::italics() applies a 25-degree horizontal vertex skew, which is a rough approximation that looks noticeably different from a real italic typeface. These defaults are fine when no custom fonts are available, but when users have access to proper Bold/Italic font files, there is no way to tell egui to use them for strong/italic markup.

Changes

Two new Option<FontFamily> fields on Style:

• strong_font: When set, RichText::strong() renders with this font family instead of applying the strong text color. The color override is skipped because the heavier strokes of a real bold typeface provide visual distinction on their own.

• emphasis_font: When set, RichText::italics() renders with this font family instead of applying the vertex skew. The skew is skipped because the real italic letterforms are used instead.

Both default to None, so existing behavior is completely unchanged.

Implementation details

The logic lives in RichText::into_text_and_format() and RichText::get_text_color() in widget_text.rs:

• into_text_and_format() resolves the bold/italic font family overrides before building the FontId. If both strong_font and emphasis_font are set and the text is both strong and italic, emphasis_font wins (since a FontId can only reference one family). Users who need combined bold-italic can register a dedicated BoldItalic family and apply it via RichText::family().

• get_text_color() skips the strong color override when strong_font is set, falling through to the default text color instead.

Usage

// 1. Register bold/italic fonts
let mut fonts = egui::FontDefinitions::default();
fonts.font_data.insert(
    "MyFont-Bold".to_owned(),
    egui::FontData::from_static(include_bytes!("MyFont-Bold.ttf")).into(),
);
fonts.families.insert(
    egui::FontFamily::Name("Bold".into()),
    vec!["MyFont-Bold".to_owned()],
);
ctx.set_fonts(fonts);

// 2. Configure style
ctx.global_style_mut(|style| {
    style.strong_font = Some(egui::FontFamily::Name("Bold".into()));
    // style.emphasis_font = Some(egui::FontFamily::Name("Italic".into()));
});

After this, any RichText::new("hello").strong() will render using the Bold font family.

Motivation

This came up while building a markdown renderer on top of egui. Markdown has **bold** and *italic* syntax, and users expect these to render with proper bold/italic typefaces when available. Libraries like egui_commonmark use RichText::strong() and RichText::italics() internally, so configuring the style once is enough to get real bold/italic rendering throughout the whole UI without patching downstream libraries.

Checklist

• I have followed the instructions in the PR template
• cargo fmt passes
• cargo clippy passes
• All 175 egui tests pass (including the new doc tests)
• No changes to default behavior (both fields default to None)
• Doc comments with working examples on both new fields

[github-actions]

Preview available at https://egui-pr-preview.github.io/pr/7946-featurestrong-emphasis-fonts
Note that it might take a couple seconds for the update to show up after the preview_build workflow has completed.

View snapshot changes at kitdiff

[Tristan117]

This looks like a great small quality of life feature!
In my application as a workaround I'm currently manually using an italic font on italic text rather than RichText::italics() which this PR would avoid.

@emilk is there a reason the italic and bold versions of the UbuntuLight default font aren't included in the default fonts (or behind a feature if binary size/performance is a concern for some targets)? Would a PR along these lines be of interest?

[emilk]

is there a reason the italic and bold versions of the UbuntuLight default font aren't included in the default fonts

Purely to keep the size of the .wasm blob smaller. Having it optional behind a feature flag would be nice.

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Something has gone wrong with the rebasing of this PR - the diff is huge, with a lot of unrelated changes

[w4nderlust]

@emilk sorry baout it. Force-pushed the branch to drop the unrelated changes; PR now contains only the two commits (the API addition + a one-line doc-link fix, 2 files, +135/-7), and I'll do the bundled-font feature flag as a follow-up PR if you think it's useful.