fix(frontend): sprint 3 post-review — real dedup test + Apply 0 guard + Link stopPropagation

- MitreTechniquesField test: rewrite dedup test to actually exercise picker
  selection path — types query, waits for option, fires pointerDown,
  asserts no PATCH sent (dedup guard in handleSelect now truly covered)
- MitreMatrixModal: Apply button disabled only when totalSelected === 0
  AND initialSelection.length === 0 (no-op case); when totalSelected === 0
  but initialSelection was non-empty, shows "Clear all" and stays enabled
  so user can explicitly wipe the list
- MitreMatrixModal tests: update disabled test to match "Clear all" label,
  add "Clear all" enabled + onApply([]) path test
- SimulationList: stopPropagation on Name <Link> to prevent double-navigate
  with row onClick handler

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/components/MitreMatrixModal.tsx

@@ -333,9 +333,11 @@ export function MitreMatrixModal({
             type="button"
             className="btn-primary"
             onClick={handleApply}
-            disabled={isLoading || isError}
+            disabled={isLoading || isError || (totalSelected === 0 && initialSelection.length === 0)}
           >
-            Apply {totalSelected > 0 ? `${totalSelected} technique${totalSelected !== 1 ? 's' : ''}` : ''}
+            {totalSelected === 0
+              ? 'Clear all'
+              : `Apply ${totalSelected} technique${totalSelected !== 1 ? 's' : ''}`}
           </button>
         </div>
       </div>

frontend/src/components/SimulationList.tsx

@@ -90,6 +90,7 @@ export function SimulationList({ engagementId }: SimulationListProps): JSX.Eleme
                   <Link
                     to={`/engagements/${engagementId}/simulations/${sim.id}/edit`}
                     className="text-ink font-medium hover:underline"
+                    onClick={(e) => e.stopPropagation()}
                   >
                     {sim.name}
                   </Link>

frontend/tests/MitreMatrixModal.test.tsx

@@ -189,6 +189,41 @@ describe('MitreMatrixModal', () => {
     });
   });
 
+  it('Apply button is disabled when no techniques selected and no initial selection', async () => {
+    renderWithProviders(
+      <MitreMatrixModal isOpen initialSelection={[]} onApply={vi.fn()} onCancel={vi.fn()} />,
+    );
+
+    await waitFor(() => screen.getByText('T1078'));
+
+    // Label is "Clear all" when totalSelected === 0, but it's disabled when initialSelection is also empty
+    const applyBtn = screen.getByRole('button', { name: /Clear all/i });
+    expect(applyBtn).toBeDisabled();
+  });
+
+  it('Apply button shows "Clear all" and stays enabled when initial selection is deselected', async () => {
+    const onApply = vi.fn();
+    const user = userEvent.setup();
+
+    renderWithProviders(
+      <MitreMatrixModal isOpen initialSelection={SELECTION} onApply={onApply} onCancel={vi.fn()} />,
+    );
+
+    await waitFor(() => screen.getByText('T1078'));
+
+    // Deselect T1078 (it was pre-selected)
+    const t1078Btn = screen.getAllByRole('button').find(
+      (btn) => btn.textContent?.includes('T1078') && !btn.getAttribute('aria-label'),
+    );
+    await user.click(t1078Btn!);
+
+    // Button should show "Clear all" and be enabled (user explicitly clearing the list)
+    const applyBtn = screen.getByRole('button', { name: /Clear all/i });
+    expect(applyBtn).not.toBeDisabled();
+    await user.click(applyBtn);
+    expect(onApply).toHaveBeenCalledWith([]);
+  });
+
   it('backdrop click calls onCancel', async () => {
     const onCancel = vi.fn();
     const user = userEvent.setup();

frontend/tests/MitreTechniquesField.test.tsx

@@ -110,16 +110,29 @@ describe('MitreTechniquesField', () => {
     expect(screen.getByRole('combobox')).toBeInTheDocument();
   });
 
-  it('dedup: adding an already-present technique does not PATCH', async () => {
+  it('dedup: selecting an already-present technique does not PATCH', async () => {
     mock.onGet('/mitre/techniques').reply(200, [T1059]);
     const user = userEvent.setup();
     renderWithProviders(
       <MitreTechniquesField value={[T1059]} simulationId={7} engagementId={42} />,
     );
-    // open picker
+
+    // Open the quick-search picker
     await user.click(screen.getByRole('button', { name: /Quick search/i }));
-    // Picker shows; but we can't easily select the same item without triggering real debounce in this test.
+    const combobox = screen.getByRole('combobox');
-    // Instead just verify no PATCH happened yet — dedup is the key invariant.
+    expect(combobox).toBeInTheDocument();
+
+    // Type to trigger the search (debounce is 200ms but fake timers not needed — mock responds immediately)
+    await user.type(combobox, 'T1059');
+
+    // Wait for the option to appear in the listbox
+    const option = await screen.findByRole('option', { name: /T1059/i });
+    expect(option).toBeInTheDocument();
+
+    // Select it via pointerDown (mirrors the component's onPointerDown handler)
+    await user.pointer({ target: option, keys: '[MouseLeft>]' });
+
+    // Dedup guard should have fired — no PATCH should have been sent
     expect(mock.history.patch.length).toBe(0);
   });