Text warps: bands with fewer lines keep the fuller band's text height

Multi-band fill warps (Deflate-Inflate, Button Pour) map a box as tall as
the tallest band's ink onto every band, centred on the band's own ink, as
PowerPoint does: a band holding one line next to a band of two keeps the
line at the same size instead of stretching it over the whole band.

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

crates/geom/src/warp.rs

@@ -4,7 +4,8 @@
 //! Text lines are shared out over the bands in order (the first ⌈n/k⌉ lines to the first band
 //! and so on). A band either
 //! - stretches its lines between a top and a bottom guide ([`Band::Fill`]): the ink box of the
-//!   band's lines fills the band vertically, and horizontally the text is scaled so that the
+//!   band's lines fills the band vertically (a band whose ink is shorter than the tallest band's
+//!   keeps the same scale, centred), and horizontally the text is scaled so that the
 //!   widest band is filled along the mean length of its guides (one scale for the whole text), or
 //! - sets its lines at their natural size along one guide ([`Band::Follow`], arches and circles):
 //!   glyphs keep their size, are placed by arc length (aligned as in the paragraph, the whole
@@ -658,6 +659,11 @@ impl Warp {
                 });
             }
         }
+        // Every fill band maps a box as tall as the tallest band's ink, centred on its own ink, so
+        // a band with fewer lines keeps them at the same size, centred between its guides.
+        let tallest = band_y.iter().flatten().map(|(y0, y1)| y1 - y0).fold(0.0, f64::max);
+        let band_y: Vec<Option<(f64, f64)>> =
+            band_y.iter().map(|r| r.map(|(y0, y1)| ((y0 + y1 - tallest) / 2.0, (y0 + y1 + tallest) / 2.0))).collect();
         for (b, band) in self.bands.iter().enumerate() {
             if let Band::Fill { top, bottom } = band {
                 let half = lines

crates/geom/src/warp/tests.rs

@@ -228,6 +228,15 @@ fn deflate_inflate_maps_each_band_separately() {
     assert!((m.map(0, Point::new(100.0, 40.0)).y / H - 0.45).abs() < 0.004);
     assert!((m.map(1, Point::new(100.0, 50.0)).y / H - 0.55).abs() < 0.004);
     assert!((m.map(1, Point::new(300.0, 80.0)).y / H - 1.0).abs() < 0.004);
+    // A band with fewer lines keeps them at the size of the fuller band, centred in its band:
+    // two lines (ink 10..60) in band 1, one line (ink 70..80) in band 2 of three.
+    let a = WarpLine { ink: Some(Rect::new(100.0, 10.0, 300.0, 30.0)), extent: (100.0, 300.0), ref_y: 28.0 };
+    let b = WarpLine { ink: Some(Rect::new(100.0, 40.0, 300.0, 60.0)), extent: (100.0, 300.0), ref_y: 58.0 };
+    let c = WarpLine { ink: Some(Rect::new(100.0, 70.0, 300.0, 80.0)), extent: (100.0, 300.0), ref_y: 78.0 };
+    let m = w.mapper(&[a, b, c], rect());
+    let (top, bot) = (m.map(2, Point::new(200.0, 70.0)).y / H, m.map(2, Point::new(200.0, 80.0)).y / H);
+    // Band 2 runs from 0.4 to 1 at the centre; the line takes the middle fifth of it.
+    assert!((top - 0.64).abs() < 0.005 && (bot - 0.76).abs() < 0.005, "{top} {bot}");
     // A shorter first line keeps its share of the width.
     let short = WarpLine { ink: Some(Rect::new(150.0, 10.0, 250.0, 40.0)), extent: (150.0, 250.0), ref_y: 34.0 };
     let m = w.mapper(&[short, l2], rect());