fix(dimension): custom arrow blocks keep their polyline width

The width of a wide polyline lives on the wire as a GPU band, which the
custom-arrow extraction dropped, so the arrow drew as a hairline.

Fixes #1130

Co-Authored-By: Claude Opus 5.5 <[email protected]>

src/scene/convert/tessellate.rs

@@ -2658,12 +2658,19 @@ fn append_custom_arrow_leaf(
         true,
     );
     for wire in wires {
-        append_custom_wire_points(
-            &wire.points,
-            &wire.points_low,
-            codec::types::Vector3::ZERO,
-            lines,
-        );
+        // A wide polyline's width rides on the wire as a GPU band; an arrow
+        // keeps only geometry, so turn the band into fill or it draws hairline.
+        // (#1130)
+        if wire.world_width > 0.0 || wire.taper_widths.iter().any(|width| *width > 0.0) {
+            append_custom_band(&wire, fill);
+        } else {
+            append_custom_wire_points(
+                &wire.points,
+                &wire.points_low,
+                codec::types::Vector3::ZERO,
+                lines,
+            );
+        }
         append_custom_fill_points(
             &wire.fill_tris,
             &wire.fill_tris_low,
@@ -2760,6 +2767,42 @@ fn append_custom_hatch_geometry(
     );
 }
 
+/// Triangles covering a wide polyline's band, one quad per segment.
+// ponytail: no join wedges at corners; add miters if a thick arrow shows notches.
+fn append_custom_band(wire: &WireModel, fill: &mut Vec<[f32; 3]>) {
+    let at = |index: usize| {
+        let point = wire.points[index];
+        let low = wire.points_low.get(index).copied().unwrap_or([0.0; 3]);
+        (!point[0].is_nan()).then(|| {
+            [
+                point[0] as f64 + low[0] as f64,
+                point[1] as f64 + low[1] as f64,
+                point[2] as f64 + low[2] as f64,
+            ]
+        })
+    };
+    let half = |index: usize| {
+        wire.taper_widths.get(index).copied().unwrap_or(wire.world_width).max(0.0) as f64 * 0.5
+    };
+    for index in 0..wire.points.len().saturating_sub(1) {
+        let (Some(a), Some(b)) = (at(index), at(index + 1)) else {
+            continue;
+        };
+        let (dx, dy) = (b[0] - a[0], b[1] - a[1]);
+        let length = dx.hypot(dy);
+        if length <= 0.0 {
+            continue;
+        }
+        let (nx, ny) = (-dy / length, dx / length);
+        let (ha, hb) = (half(index), half(index + 1));
+        let corner = |p: [f64; 3], h: f64, side: f64| {
+            [(p[0] + nx * h * side) as f32, (p[1] + ny * h * side) as f32, p[2] as f32]
+        };
+        let quad = [corner(a, ha, 1.0), corner(a, ha, -1.0), corner(b, hb, -1.0), corner(b, hb, 1.0)];
+        fill.extend([quad[0], quad[1], quad[2], quad[0], quad[2], quad[3]]);
+    }
+}
+
 fn append_custom_wire_points(
     points: &[[f32; 3]],
     points_low: &[[f32; 3]],