T17: a font saves, and a settings file starts, only when gpui loads it as the terminal draws it, so the terminal never falls back without a word; numbers save on Enter, on leaving the field and on each step; after a refused change a number field drops its draft and the font picker shows the saved family again

tests/fonts.rs

@@ -0,0 +1,17 @@
+//! gpui builds its Mac platform only on the main thread, so this test brings its own main.
+
+#[cfg(target_os = "macos")]
+fn main() {
+    use elyterminal::term::load_font;
+    let system = gpui::TextSystem::new(gpui_platform::current_platform(true).text_system());
+    assert!(load_font(&gpui::font("Menlo"), &system).is_ok());
+    // A family missing, and one gpui passes over for lacking an `m`.
+    for family in ["No Such Family", "Apple Color Emoji"] {
+        let error = load_font(&gpui::font(family), &system).expect_err(family);
+        assert!(error.to_string().contains(family), "{error}");
+    }
+    println!("fonts: a font gpui does not load is refused");
+}
+
+#[cfg(not(target_os = "macos"))]
+fn main() {}