Emit pane aria-controls only while the pane is mounted

src/app/components/PaneToggleStrip.test.tsx

@@ -56,6 +56,18 @@ describe('PaneToggleStrip', () => {
     expect(library.getAttribute('aria-label')).toBe('Library pane');
   });
 
+  test('aria-controls points at the pane only while it is shown (no dangling IDREF)', () => {
+    // App unmounts a hidden pane's <section>, so emitting aria-controls then would
+    // leave the IDREF pointing at nothing. It must appear only when pressed/shown.
+    const [library] = toggleButtons();
+    expect(library.hasAttribute('aria-controls')).toBe(true);
+
+    act(() => library.click()); // hide the pane
+
+    expect(library.getAttribute('aria-pressed')).toBe('false');
+    expect(library.hasAttribute('aria-controls')).toBe(false);
+  });
+
   test('roving tabindex: exactly one control is in the tab order at a time', () => {
     const all = [...toggleButtons(), datasetsButton()];
     expect(all.filter((b) => b.tabIndex === 0)).toHaveLength(1);

src/app/components/PaneToggleStrip.tsx

@@ -102,11 +102,13 @@ export function PaneToggleStrip() {
           type="button"
           aria-pressed={visible[item.pane]}
           aria-label={item.label}
-          // TODO: when a pane is hidden, App unmounts its <section>, so this
-          // aria-controls IDREF dangles until the pane is shown again. Harmless
-          // (AT ignores unresolved IDREFs) but technically invalid — consider
-          // keeping the section mounted-but-hidden, or dropping aria-controls.
-          aria-controls={item.controls}
+          // Only point at the pane while it's actually mounted: App unmounts a
+          // hidden pane's <section>, so emitting aria-controls then would leave a
+          // dangling IDREF (invalid, even if AT ignores it). The association is
+          // meaningful only when the target exists, and the aria-pressed state
+          // already conveys hidden/shown either way (contract: arch/10 → toggle
+          // strip — the bullet lists aria-pressed + a stable name, not controls).
+          aria-controls={visible[item.pane] ? item.controls : undefined}
           // Description (not the name): hints what activating does. The name stays
           // stable (aria-label) per APG's toggle-button rule.
           title={`${visible[item.pane] ? 'Hide' : 'Show'} ${item.label.toLowerCase()}`}