Retain layout sizes by pixel axis

tests/retained.rs

@@ -191,6 +191,17 @@ impl Widget for ReadsOutput {
     }
 }
 
+struct ReadsWidth {
+    draws: Rc<Cell<usize>>,
+}
+
+impl Widget for ReadsWidth {
+    fn draw(&mut self, painter: &mut Painter) -> Size {
+        self.draws.set(self.draws.get() + 1);
+        Size::abs((painter.output_len(Axis::X) / 4.0, 20.0).into())
+    }
+}
+
 #[test]
 fn a_resize_does_not_redraw_what_the_shader_can_move() {
     let mut h = Harness::new((400, 200));
@@ -227,6 +238,65 @@ fn a_resize_redraws_what_read_the_output() {
     assert_eq!(draws.get(), settled + 1);
 }
 
+#[test]
+fn a_resize_only_redraws_read_output_axes() {
+    let mut h = Harness::new((400, 200));
+    let draws = Rc::new(Cell::new(0));
+    let leaf = ReadsWidth {
+        draws: draws.clone(),
+    }
+    .add(&mut h.rsc);
+    h.set_root(leaf);
+    let settled = draws.get();
+
+    h.resize((400, 300));
+    h.frame();
+    assert_eq!(draws.get(), settled, "height was never read");
+
+    h.resize((800, 300));
+    h.frame();
+    assert_eq!(draws.get(), settled + 1, "width changes its answer");
+}
+
+#[test]
+fn subpixel_resize_changes_accumulate_from_the_last_layout() {
+    let mut h = Harness::new((400, 200));
+    let draws = Rc::new(Cell::new(0));
+    let leaf = ReadsWidth {
+        draws: draws.clone(),
+    }
+    .add(&mut h.rsc);
+    h.set_root(leaf);
+    let settled = draws.get();
+
+    for width in [400.02, 400.04, 400.05] {
+        h.resize((width, 200.0));
+        h.frame();
+        assert_eq!(draws.get(), settled);
+    }
+
+    h.resize((400.06, 200.0));
+    h.frame();
+    assert_eq!(draws.get(), settled + 1);
+}
+
+#[test]
+fn subpixel_box_changes_accumulate_from_the_last_draw() {
+    let mut h = Harness::new((400, 200));
+    let (first, draws, _) = pair(&mut h, OnResize::Redraw);
+    let settled = draws.get();
+
+    for width in [100.02, 100.04, 100.05] {
+        h.rsc[first].size.x = Len::abs(width);
+        h.frame();
+        assert_eq!(draws.get(), settled);
+    }
+
+    h.rsc[first].size.x = Len::abs(100.06);
+    h.frame();
+    assert_eq!(draws.get(), settled + 1);
+}
+
 #[test]
 fn reporting_the_same_output_size_does_not_start_a_resize() {
     let mut h = Harness::new((400, 200));