fix(ui): a caption never opens under a finger

On an iPad a tap's synthetic mouseenter opened the Tooltip; WebKit then took the
tap as a hover and swallowed its click, so every rail button needed two taps,
and with no mouseleave from a finger the caption stayed up. Tooltip now opens
for a mouse and a key's focus only (owner: no captions on touch).

frontend/src/components/ui/Tooltip.layer.dom.test.tsx

@@ -64,3 +64,41 @@ describe("an open tooltip", () => {
     await waitFor(() => expect(screen.queryByRole("tooltip")).toBeNull());
   });
 });
+
+// On an iPad a tap's synthetic mouseenter showed the caption; WebKit then took the tap as a hover and
+// swallowed its click (every rail button needed two taps), and with no mouseleave the caption stayed.
+describe("a tooltip under a finger", () => {
+  class PointerEventStub extends MouseEvent {
+    pointerType: string;
+    constructor(type: string, init: PointerEventInit = {}) {
+      super(type, init);
+      this.pointerType = init.pointerType ?? "";
+    }
+  }
+  const hover = (trigger: HTMLElement, pointerType: string) =>
+    act(() => {
+      trigger.dispatchEvent(new PointerEventStub("pointerdown", { bubbles: true, pointerType }));
+      trigger.dispatchEvent(new PointerEventStub("pointerenter", { pointerType }));
+      fireEvent.mouseEnter(trigger);
+      fireEvent.mouseMove(trigger);
+    });
+  const mount = () => {
+    render(
+      <Tooltip hint="A hint" always>
+        <button>icon</button>
+      </Tooltip>,
+    );
+    return screen.getByText("icon").parentElement as HTMLElement;
+  };
+
+  it("does not open from a tap", async () => {
+    hover(mount(), "touch");
+    await new Promise((r) => setTimeout(r, 50));
+    expect(screen.queryByRole("tooltip")).toBeNull();
+  });
+
+  it("still opens under a mouse", async () => {
+    hover(mount(), "mouse");
+    expect(await screen.findByRole("tooltip")).toBeTruthy();
+  });
+});