iris: add replaceable glyph atlas buckets

src/layout_tests.rs

@@ -362,6 +362,33 @@ fn scrolling_moves_in_o1_without_a_redraw() {
     assert_eq!(moves, 1, "the scrolled subtree should move in one write");
 }
 
+#[test]
+fn fractional_scrolling_keeps_primitive_edges_on_physical_pixels() {
+    let mut rsc = TestRsc { ui: Ui::default() };
+    let (scroll, root, rects) = scrolled_rects(&mut rsc, 20);
+    let mut render = UiRenderState::new();
+    render.resize((800.0, 600.0));
+
+    render.update(&root, &mut rsc);
+    rsc.ui.widgets.get_mut(&scroll).unwrap().scroll(0.0);
+    render.update(&root, &mut rsc);
+    rsc.ui.widgets.get_mut(&scroll).unwrap().scroll(-37.25);
+    render.update(&root, &mut rsc);
+
+    let slot = render
+        .first_primitive(rects[2].id())
+        .expect("the scrolled rect has a primitive");
+    let corners = render.primitive_corners(slot, &rsc);
+    for edge in [
+        corners.top_left.x,
+        corners.top_left.y,
+        corners.bot_right.x,
+        corners.bot_right.y,
+    ] {
+        assert_eq!(edge.fract(), 0.0, "fractional rendered edge: {corners:?}");
+    }
+}
+
 #[test]
 fn hit_testing_follows_a_scrolled_widget() {
     let mut rsc = TestRsc { ui: Ui::default() };