Avoid speculative layout when retained answers suffice

tests/retained.rs

@@ -79,10 +79,9 @@ fn a_leaf_that_depends_on_its_box_is_drawn_again_when_the_box_changes() {
     h.rsc[first].size = Size::from((150, 200));
     h.frame();
 
-    // Twice: once for the span to measure it, once for its real box. A child
-    // that can hint its length is spared the first, and a smaller number here
-    // means someone has made that cheaper rather than broken it.
-    assert_eq!(draws.get(), settled + 2);
+    // The preceding fixed child makes the remaining box this child's real
+    // box, so measuring it also draws it in its final place.
+    assert_eq!(draws.get(), settled + 1);
     assert_corners!(h, second, (150, 0), (400, 200));
 }
 
@@ -116,6 +115,50 @@ fn a_span_relays_out_when_a_child_it_measured_changes() {
     assert_corners!(h, second, (250, 0), (400, 200));
 }
 
+#[test]
+fn a_repaint_that_keeps_its_size_does_not_relay_out() {
+    let mut h = Harness::new((400, 200));
+    let (first, draws) = counted(&mut h, Size::from((100, 200)), OnResize::Translate);
+    let (second, _) = counted(&mut h, Size::REST, OnResize::Translate);
+    h.set_root((first, second).span(Dir::RIGHT));
+    let settled = draws.get();
+
+    // Taking mutable access is the ordinary content-change signal. This
+    // widget returns the same size, so the parent has nothing to lay out.
+    let _ = h.rsc.widgets_mut().get_dyn_mut(first.id());
+    h.frame();
+
+    assert_eq!(draws.get(), settled + 1);
+}
+
+#[test]
+fn scrolling_reuses_the_clean_contents_size() {
+    let mut h = Harness::new((400, 200));
+    let (inner, draws) = counted(&mut h, Size::from((400, 600)), OnResize::Translate);
+    let scroll = Scroll::new(inner.add_strong(&mut h.rsc), Axis::Y).add(&mut h.rsc);
+    h.set_root(scroll);
+    let settled = draws.get();
+
+    h.rsc[scroll].scroll(40.0);
+    h.frame();
+
+    assert_eq!(draws.get(), settled);
+    assert_corners!(h, inner, (0, -360), (400, 240));
+}
+
+#[test]
+fn scrolling_remeasures_changed_contents() {
+    let mut h = Harness::new((400, 200));
+    let (inner, _) = counted(&mut h, Size::from((400, 600)), OnResize::Translate);
+    let scroll = Scroll::new(inner.add_strong(&mut h.rsc), Axis::Y).add(&mut h.rsc);
+    h.set_root(scroll);
+
+    h.rsc[inner].size = Size::from((400, 800));
+    h.frame();
+
+    assert_corners!(h, inner, (0, -600), (400, 200));
+}
+
 #[test]
 fn a_placed_child_survives_the_next_frame() {
     let mut h = Harness::new((400, 200));