Fonts: a letter with combining marks the run's font lacks takes a face that has them all

"Z̴̡̛͓a̸̧̘l̵̩g̶̞o̸̤" in an Aptos run drew its marks as missing-glyph boxes: no face in the chain
covered the whole cluster and the fallback for its base letter neither, so the run's own font was
kept. PowerPoint draws the marks stacked. A loaded plain face that covers the whole cluster is
now taken before falling back to the run's font. w09-scripts s1 8.40 -> 7.86.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

crates/fonts/src/fallback.rs

@@ -185,7 +185,10 @@ impl FontDb {
         let f = match self.fallback_for(key, exclude) {
             Some(f) if covers_cluster(&f, cluster) => f,
             Some(f) if chain.iter().all(|c| !c.covers(key)) => f,
-            _ => first(),
+            // A letter with marks the run's font lacks (stacked combining marks): a loaded plain
+            // face that draws the whole cluster, as PowerPoint draws the marks, before the run's
+            // font with missing-glyph boxes.
+            _ => self.read_faces().iter().find(|f| !f.is_color() && covers_cluster(f, cluster)).cloned().unwrap_or_else(first),
         };
         // A pictograph that defaults to text presentation (⚠ ♥ ⬆ ▶) whose first fallback is the
         // colour emoji font: PowerPoint draws it from a plain symbol font.

crates/text/src/tests.rs

@@ -1196,3 +1196,23 @@ fn rtl_columns_start_on_the_right_and_just_anchoring_sits_at_the_bottom() {
         assert!((lay(&p, &sh, 400.0, 300.0).lines[0].top - b).abs() < 0.01, "{a:?}");
     }
 }
+
+#[test]
+fn stacked_combining_marks_take_a_face_that_draws_them() {
+    // A letter with marks the run's font lacks draws in a face that has them all (no boxes).
+    let db = FontDb::global();
+    let cluster = ['Z', '\u{0334}', '\u{0321}', '\u{031B}'];
+    let run_face = db.face("Inter", "Regular");
+    let f = db.cascade(&cluster, std::slice::from_ref(&run_face), "Regular");
+    if !deckcraft_fonts::covers_cluster(&run_face, &cluster) && ["DejaVu Sans", "Segoe UI"].iter().any(|fam| db.has_family(fam)) {
+        // Make sure a face with the marks is loaded, then the cluster must take one.
+        for fam in ["DejaVu Sans", "Segoe UI"] {
+            if db.has_family(fam) {
+                let _ = db.face(fam, "Regular");
+            }
+        }
+        let f = db.cascade(&cluster, std::slice::from_ref(&run_face), "Regular");
+        assert!(deckcraft_fonts::covers_cluster(&f, &cluster), "{} lacks the marks", f.family);
+    }
+    let _ = f;
+}