Type: Invert Wrap with an offset applies to a band of no height

A band whose top and bottom are equal lost its only piece when the repeated cut was removed, so the line kept the frame's whole width. Anchor cuts now apply only to a band with height.

crates/text/src/layout/region.rs

@@ -446,7 +446,9 @@ impl Region {
                 // count.
                 let pieces = if *off > 0.0 { ((bottom - top) * 4.0 / off).ceil().clamp(1.0, INVERT_WRAP_PIECES) } else { 1.0 };
                 let mut cuts: Vec<f64> = (0..=pieces as usize).map(|k| top + (bottom - top) * k as f64 / pieces).collect();
-                if *off > 0.0 {
+                // A band of no height stays one piece: no anchor lies strictly inside it, and
+                // removing the repeated cut would leave no piece.
+                if *off > 0.0 && bottom > top {
                     let mut anchors: Vec<f64> =
                         polys.iter().flatten().flat_map(|p| [p.y - off, p.y + off]).filter(|&y| y > top && y < bottom).collect();
                     anchors.sort_by(f64::total_cmp);
@@ -615,6 +617,11 @@ fn a_line_stays_clear_of_a_wrap_object_that_reaches_into_its_band() {
     let path = PathData::from_bezpath(&closed(&[upper, lower]));
     let wrap = vectorcraft_doc::WrapShape { path, wrap: vectorcraft_doc::TextWrap { offset: 5.0, invert: true } };
     assert!(Region::cells(&frame, &LayoutOptions::default(), &[wrap])[0].spans(40.0, 60.0).is_empty());
+    // A band of no height keeps Invert Wrap and its offset.
+    let strip: &[(f64, f64)] = &[(20.0, 0.0), (40.0, 0.0), (40.0, 100.0), (20.0, 100.0)];
+    let path = PathData::from_bezpath(&closed(&[strip]));
+    let wrap = vectorcraft_doc::WrapShape { path, wrap: vectorcraft_doc::TextWrap { offset: 5.0, invert: true } };
+    assert_spans(&Region::cells(&frame, &LayoutOptions::default(), &[wrap])[0].spans(50.0, 50.0), &[(15.0, 45.0)]);
 }
 
 /// A hole that starts and ends inside a line's band splits the line, and a band below the hole, or