Color Wheel: a step past the edge of the square leaves the color as it is

An arrow key or a screen reader's step outward at the edge of the
square returned the clamped point, which equals the current one, and the
wheel picked it again. For a CMYK fill the pick separated the color
again, so one step at 100% saturation turned C100 into C71 M16 and added
an undo step. color_field now returns no point when the clamped point is
unchanged; with no marker a key or an action still sets a color.

crates/ui-egui/src/panels/tests_color_wheel.rs

@@ -430,6 +430,21 @@ fn screen_readers_read_and_move_the_ring_and_the_square() {
     assert!(near(hsb(&fill(h.state())), [200.0, 0.19, 0.6], [1e-3, 1e-3, 1e-3]), "{:?}", hsb(&fill(h.state())));
 }
 
+/// A step past the edge of the square leaves the fill alone: a CMYK cyan shows at 100% saturation,
+/// and a step up from there neither separates it again nor adds an undo step.
+#[test]
+fn a_step_past_the_edge_of_the_square_changes_nothing() {
+    let mut h = harness(app(json!({"c": 1.0, "m": 0.0, "y": 0.0, "k": 0.0})));
+    let before = fill(h.state());
+    assert_eq!(hsb(&before)[1], 1.0, "{:?}", hsb(&before));
+    let undo = undo_len(h.state());
+    let (target_node, target_tree) = h.get_by_label("Saturation and brightness").accesskit_node().locate();
+    h.event(Event::AccessKitActionRequest(ActionRequest { target_node, target_tree, action: Action::Increment, data: None }));
+    h.run_steps(1);
+    assert_eq!(fill(h.state()), before);
+    assert_eq!(undo_len(h.state()), undo, "no undo step");
+}
+
 /// The Color Picker's color field is the same widget: a press focuses it, the arrows step the
 /// components it shows, and screen readers read its name and both components and step the first.
 #[test]

crates/ui-egui/src/widgets/color.rs

@@ -378,7 +378,8 @@ fn focus_outline(ui: &Ui, resp: &Response, rect: Rect) {
 /// readers read `name` and, when `marked`, both components. They read it as a slider over
 /// `axes[0]`'s units (0 when not `marked`), which their increment and decrement actions move as →
 /// and ← do and their set-value action sets, as for a [`hue_ring`]. Returns the clicked or dragged
-/// point, or the point the keys or actions moved to, and the phase.
+/// point, or the point the keys or actions moved to, and the phase. When `marked`, a step past an
+/// edge returns no point.
 pub fn color_field(
     ui: &mut Ui,
     id: impl std::hash::Hash,
@@ -435,9 +436,11 @@ pub fn color_field(
         return (Some((f(p.x, rect.left(), rect.width()), f(p.y, rect.top(), rect.height()))), phase);
     }
     let across = set.map_or(x + (dx + steps) / axes[0].units, |v| v / axes[0].units);
-    // Without a marker there is no color yet, so a set value applies even when it equals `x`.
-    if across != x || dy != 0.0 || (set.is_some() && !marked) {
-        let moved = (across.clamp(0.0, 1.0), (y - dy / axes[1].units).clamp(0.0, 1.0));
+    let moved = (across.clamp(0.0, 1.0), (y - dy / axes[1].units).clamp(0.0, 1.0));
+    // A step past an edge leaves the point where it is. Without a marker there is no color yet, so
+    // a key or an action applies even when the point stays.
+    let asked = set.is_some() || dx + steps != 0.0 || dy != 0.0;
+    if moved != (x.clamp(0.0, 1.0), y.clamp(0.0, 1.0)) || (asked && !marked) {
         return (Some(moved), Live::Released);
     }
     (None, Live::Idle)