address review: reversed run ranges take the scan

crates/text/src/layout.rs

@@ -628,10 +628,10 @@ fn is_hanging_open(c: char) -> bool {
     matches!(c, '"' | '\'' | '\u{2018}' | '\u{201c}' | '(' | '\u{ab}')
 }
 
-/// Whether each run starts at or after the end of the one before it (the runs of a document):
-/// the precondition of [`run_at`]'s bisection.
+/// Whether no run is reversed and each starts at or after the end of the one before it (the runs
+/// of a document): the precondition of [`run_at`]'s bisection.
 fn runs_ordered(runs: &[(Range<usize>, TextStyle)]) -> bool {
-    runs.windows(2).all(|w| w[0].0.end <= w[1].0.start)
+    runs.iter().all(|(r, _)| r.start <= r.end) && runs.windows(2).all(|w| w[0].0.end <= w[1].0.start)
 }
 
 /// Style runs with whether they are [`runs_ordered`].

crates/text/src/layout/tests.rs

@@ -522,5 +522,12 @@ fn run_lookup_by_bisection_matches_the_scan() {
         assert_eq!(run_at(&overlapping, false, b), scan(&overlapping, b), "{b}");
     }
     assert_eq!(run_at(&overlapping, false, 6), 0);
+    // A reversed range (20..0) among others is not ordered either, and takes the scan.
+    #[allow(clippy::reversed_empty_ranges)]
+    let reversed: Vec<(Range<usize>, TextStyle)> = vec![(0..10, st(10.0)), (20..0, st(20.0)), (5..15, st(30.0)), (15..20, st(40.0))];
+    assert!(!runs_ordered(&reversed));
+    for b in 0..30 {
+        assert_eq!(run_at(&reversed, runs_ordered(&reversed), b), scan(&reversed, b), "{b}");
+    }
     assert_eq!(run_at(&[], true, 5), 0);
 }