Add regression test for variable font weight matching

tests/variable_font_weight.rs

@@ -0,0 +1,50 @@
+use std::path::PathBuf;
+
+use cosmic_text::{fontdb, Attrs, Buffer, Family, FontSystem, Metrics, Shaping, Weight};
+
+/// Variable fonts must be matched at all weights within their `wght` axis
+/// range, not just the default weight they register at in fontdb, otherwise
+/// they will fall back to a system font despite being able to provide the
+/// requested weight.
+#[test]
+fn variable_font_all_weights_match() {
+    let repo_dir = std::env::var("CARGO_MANIFEST_DIR").unwrap();
+    let fonts_path = PathBuf::from(&repo_dir).join("fonts");
+
+    let mut font_system = FontSystem::new();
+    font_system
+        .db_mut()
+        .load_font_data(std::fs::read(fonts_path.join("InterVariable.ttf")).unwrap());
+
+    for w in [100, 200, 300, 400, 500, 600, 700, 800, 900] {
+        let metrics = Metrics::new(16.0, 20.0);
+        let mut buffer = Buffer::new(&mut font_system, metrics);
+
+        let glyph_font_ids: Vec<fontdb::ID>;
+        {
+            let mut buffer = buffer.borrow_with(&mut font_system);
+            let attrs = Attrs::new()
+                .family(Family::Name("Inter Variable"))
+                .weight(Weight(w));
+            buffer.set_size(Some(300.0), Some(100.0));
+            buffer.set_text("Hello world", &attrs, Shaping::Advanced, None);
+            buffer.shape_until_scroll(true);
+
+            glyph_font_ids = buffer
+                .layout_runs()
+                .flat_map(|run| run.glyphs.iter().map(|g| g.font_id))
+                .collect();
+        }
+
+        assert!(!glyph_font_ids.is_empty(), "Weight {w}: no glyphs produced");
+
+        for id in &glyph_font_ids {
+            let face = font_system.db().face(*id).unwrap();
+            let family = &face.families[0].0;
+            assert!(
+                family.contains("Inter"),
+                "Weight {w}: expected Inter, got \"{family}\""
+            );
+        }
+    }
+}