Add explicit orthogonal span sizing

tests/retained.rs

@@ -264,6 +264,25 @@ fn a_resize_does_not_redraw_what_the_shader_can_move() {
     assert_corners!(h, leaf, (0, 0), (800, 100));
 }
 
+#[test]
+fn a_full_ortho_span_moves_its_child_without_redrawing_it() {
+    let mut h = Harness::new((400, 200));
+    let (leaf, draws) = counted(&mut h, Size::LEFTOVER, false);
+    let span = (leaf,)
+        .span(Dir::RIGHT)
+        .ortho(OrthoSize::Full)
+        .add(&mut h.rsc);
+    h.set_root(span);
+    let settled = draws.get();
+
+    h.resize((400, 100));
+    h.frame();
+
+    assert_eq!(draws.get(), settled);
+    assert_corners!(h, leaf, (0, 0), (400, 100));
+    assert_eq!(h.render.active[&span.id()].size.y, Len::rel(1.0));
+}
+
 /// The output is the root of the box chain, so a resize is a box that changed
 /// length like any other -- there is not a second rule for the window. A
 /// drawing that holds for one length is drawn again whichever box moved.