swarm-ui: unit test the popover focus helpers

The helpers take the active element and body as arguments, so the cases
(focus into the box, keep a control that took focus, restore only when
focus was lost, find the trigger) run under node's test runner.

frontend/packages/swarm-ui/test/popover-focus.test.mjs

@@ -0,0 +1,83 @@
+import { test } from "node:test";
+import assert from "node:assert/strict";
+import { focusInto, restoreFocus, triggerOf } from "../src/ui/popover/focus.ts";
+
+// Stand-ins for DOM elements: only the members the helpers call.
+function el({ contains = () => false, first = null, matches = false } = {}) {
+  const calls = { focus: 0 };
+  return {
+    calls,
+    focus() {
+      calls.focus += 1;
+    },
+    contains,
+    matches: () => matches,
+    querySelector: () => first,
+  };
+}
+
+test("focusInto: focuses the first control in the box", () => {
+  const control = el();
+  const box = el({ first: control });
+  focusInto(box, null);
+  assert.equal(control.calls.focus, 1);
+  assert.equal(box.calls.focus, 0);
+});
+
+test("focusInto: focuses the box when it has no control", () => {
+  const box = el();
+  focusInto(box, null);
+  assert.equal(box.calls.focus, 1);
+});
+
+test("focusInto: keeps focus a control inside the box took itself", () => {
+  const inner = el();
+  const control = el();
+  const box = el({ first: control, contains: (n) => n === inner });
+  focusInto(box, inner);
+  assert.equal(control.calls.focus, 0);
+  assert.equal(box.calls.focus, 0);
+});
+
+test("focusInto: a missing box does nothing", () => {
+  focusInto(null, null);
+});
+
+test("restoreFocus: focus on body goes back to the trigger", () => {
+  const body = el();
+  const trigger = el();
+  restoreFocus(trigger, body, body);
+  assert.equal(trigger.calls.focus, 1);
+});
+
+test("restoreFocus: no focused element goes back to the trigger", () => {
+  const trigger = el();
+  restoreFocus(trigger, null, el());
+  assert.equal(trigger.calls.focus, 1);
+});
+
+test("restoreFocus: another focused control keeps focus", () => {
+  const trigger = el();
+  restoreFocus(trigger, el(), el());
+  assert.equal(trigger.calls.focus, 0);
+});
+
+test("restoreFocus: a missing trigger does nothing", () => {
+  const body = el();
+  restoreFocus(null, body, body);
+});
+
+test("triggerOf: the anchor itself when Tab reaches it", () => {
+  const anchor = el({ matches: true, first: el() });
+  assert.equal(triggerOf(anchor), anchor);
+});
+
+test("triggerOf: the first control inside a wrapper", () => {
+  const inner = el();
+  const anchor = el({ first: inner });
+  assert.equal(triggerOf(anchor), inner);
+});
+
+test("triggerOf: a missing anchor has no trigger", () => {
+  assert.equal(triggerOf(null), null);
+});