Fix arrange.distribute span to use the selection's full extent

Found while testing crates/engine/src/cmd/arrange.rs:325-327 (distribute)

Co-Authored-By: Claude Haiku 5.5 <noreply@anthropic.com>
Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

crates/engine/src/cmd/arrange.rs

@@ -322,9 +322,9 @@ fn distribute(s: &mut Session, p: &Value) -> Result<Value> {
     let (start, end) = if to_slide {
         if horiz { (0.0, size.width) } else { (0.0, size.height) }
     } else {
-        let f = boxes.first().map(|b| b.1).unwrap_or_default();
-        let l = boxes.last().map(|b| b.1).unwrap_or_default();
-        if horiz { (f.x0, l.x1) } else { (f.y0, l.y1) }
+        let lo = |f: fn(&Rect) -> f64| boxes.iter().map(|b| f(&b.1)).fold(f64::INFINITY, f64::min);
+        let hi = |f: fn(&Rect) -> f64| boxes.iter().map(|b| f(&b.1)).fold(f64::NEG_INFINITY, f64::max);
+        if horiz { (lo(|r| r.x0), hi(|r| r.x1)) } else { (lo(|r| r.y0), hi(|r| r.y1)) }
     };
     let n = boxes.len();
     let gap = if n > 1 { ((end - start) - total) / (n - 1) as f64 } else { 0.0 };

crates/engine/src/tests.rs

@@ -338,6 +338,19 @@ fn align_and_distribute() {
     assert!((x1 - 255.0).abs() < 1e-6, "{x1}");
 }
 
+#[test]
+fn distribute_wide_middle_stays_in_selection_bounds() {
+    let mut s = session();
+    let mut ids = vec![];
+    for (x, w) in [(0.0, 10.0), (20.0, 100.0), (30.0, 10.0)] {
+        ids.push(s.execute("shape.insert", &json!({"preset": "rect", "rect": [x, 10, w, 10]})).unwrap()["id"].as_u64().unwrap());
+    }
+    s.execute("edit.select", &json!({"ids": ids})).unwrap();
+    s.execute("arrange.distribute", &json!({"dir": "horizontal", "to": "selection"})).unwrap();
+    let xs: Vec<f64> = ids.iter().map(|i| s.execute("shape.inspect", &json!({"id": i})).unwrap()["box"]["x"].as_f64().unwrap()).collect();
+    assert!(xs.iter().all(|x| (*x - [0.0, 10.0, 110.0][xs.iter().position(|y| y == x).unwrap()]).abs() < 1e-6), "{xs:?}");
+}
+
 #[test]
 fn pointer_draw_move_resize() {
     let mut s = session();