fix(csr): make dialog visibility testable and cover the focus trap

The focus trap and scroll lock shipped in 0.8.5 gated on
getBoundingClientRect, which the test DOM always reports as zero, so a dialog
never counted as open and none of that behaviour ran under test. focusableWithin
had the same measurement gate and would have found no items even once the
visibility check was fixed.

Both now use the hidden attribute and the data-show marker the components
already emit. Behaviour in a real browser is unchanged; the difference is that
it is now covered.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

packages/csr/test/reactive.test.ts

@@ -993,3 +993,40 @@ test("nested roving groups do not capture the outer group items", () => {
   a.dispatchEvent(new win.KeyboardEvent("keydown", { key: "ArrowRight", bubbles: true }));
   expect(doc.activeElement!.id).toBe("b");
 });
+
+test("opening a modal dialog traps Tab inside it", () => {
+  const win = mount(
+    `<div>
+       <button id="outside">Outside</button>
+       <div data-show="true">
+         <section role="dialog" aria-modal="true" tabindex="-1">
+           <button id="first">First</button>
+           <button id="last">Last</button>
+         </section>
+       </div>
+     </div>`,
+  );
+  const doc = win.document;
+  const last = doc.querySelector("#last") as unknown as HTMLElement;
+  last.focus();
+  last.dispatchEvent(new win.KeyboardEvent("keydown", { key: "Tab", bubbles: true }));
+  expect(doc.activeElement!.id).toBe("first");
+});
+
+test("a hidden dialog does not trap Tab", () => {
+  const win = mount(
+    `<div>
+       <button id="outside">Outside</button>
+       <div data-show="false">
+         <section role="dialog" aria-modal="true" tabindex="-1">
+           <button id="first">First</button>
+         </section>
+       </div>
+     </div>`,
+  );
+  const doc = win.document;
+  const outside = doc.querySelector("#outside") as unknown as HTMLElement;
+  outside.focus();
+  outside.dispatchEvent(new win.KeyboardEvent("keydown", { key: "Tab", bubbles: true }));
+  expect(doc.activeElement!.id).toBe("outside");
+});