The idea consists of adding fallbacks to the FontTweak structure, which then get used in egui's Font (actually a wrapper over multiple FontImpl) to identify fallback characters. We look through each wrapped FontImpl, stopping in the first fallback character we found, defaulting to the empty glyph if none is found.

Example output when replacing the user font by MaterialSymbolsSharp in the egui custom font example:

impl eframe::App for MyApp {
    fn update(&mut self, ctx: &egui::Context, _frame: &mut eframe::Frame) {
        egui::CentralPanel::default().show(ctx, |ui| {
            ui.heading("\u{E834} egui using custom fonts");
            ui.text_edit_multiline(&mut self.text);
        });
    }
}

Note that as it is (mostly due to my very limited understanding of fonts in general), this isn't enough to load .notdef properly, even with a user-definied fallback character set. I suspect this however has to do with ttf_parser seemingly not actually loading .notdef.

• Closes Can't load Material Design font because of missing missing glyph #3526 (this one was technically originally closed by Don't panic when replacement glyph is not found #4542)
• See Custom default replacement glyphs for font #4642