Make alignment a widget property

tests/layout.rs

@@ -74,17 +74,39 @@ fn an_empty_widget_takes_a_share_of_a_span() {
 #[test]
 fn a_child_drawn_twice_moves_once() {
     let mut h = Harness::new((400, 200));
-    // `Aligned` draws its child twice; listing it twice would move it twice.
+    // The span measures a child and then places it; listing it twice would
+    // move it twice. The span's own fixed total is shorter than the window,
+    // so the span is centred in it and everything under it carries that.
     let inner = rect(Color::BLUE).add(&mut h.rsc);
     let centered = inner.center().width(200).add(&mut h.rsc);
     let left = rect(Color::RED).width(100).add(&mut h.rsc);
     h.set_root((left, centered).span(Dir::RIGHT));
-    assert_corners!(h, inner, (100, 0), (300, 200));
+    assert_corners!(h, inner, (150, 0), (350, 200));
 
     h.set_len(left, Axis::X, 150);
     h.frame();
 
-    assert_corners!(h, inner, (150, 0), (350, 200));
+    assert_corners!(h, inner, (175, 0), (375, 200));
+}
+
+#[test]
+fn alignment_accepts_an_arbitrary_fraction_and_changes_at_runtime() {
+    let mut h = Harness::new((400, 200));
+    let fixed = rect(Color::BLUE).sized((100, 100)).add(&mut h.rsc);
+    h.rsc
+        .widgets_mut()
+        .set_alignment(fixed, Axis::X, AxisAlign::new(0.25));
+    h.rsc
+        .widgets_mut()
+        .set_alignment(fixed, Axis::Y, AxisAlign::NEG);
+    h.set_root(fixed);
+    assert_corners!(h, fixed, (75, 0), (175, 100));
+
+    h.rsc
+        .widgets_mut()
+        .set_alignment(fixed, Axis::X, AxisAlign::new(0.75));
+    h.frame();
+    assert_corners!(h, fixed, (225, 0), (325, 100));
 }
 
 #[test]
@@ -143,15 +165,17 @@ fn a_moved_subtree_takes_its_children_with_it() {
     let first = rect(Color::RED).height(40).add(&mut h.rsc);
     let inner = rect(Color::BLUE).add(&mut h.rsc);
     let row = inner.pad(10).height(40).region_node().add(&mut h.rsc);
+    // 80 of fixed rows in a 400 window, so the span takes 80 and sits in the
+    // middle of what it was given.
     h.set_root((first, row).span(Dir::DOWN));
-    assert_corners!(h, inner, (10, 50), (390, 70));
+    assert_corners!(h, inner, (10, 210), (390, 230));
 
     h.set_len(first, Axis::Y, 80);
     h.frame();
 
     // The row opted into one movable region, so its descendants follow one
     // entry rather than having their primitive regions rewritten.
-    assert_corners!(h, inner, (10, 90), (390, 110));
+    assert_corners!(h, inner, (10, 230), (390, 250));
 }
 
 #[test]