feat(ui): hide modifier tooltip on pieces with no active modifiers

The hover tooltip previously rendered on every piece regardless of
whether it had any modifier facts, showing just a piece-type header and
'No active modifiers' — noise with zero information the user can't
already see on the board.

Now returns null when there are no modifier rows. The pinned panel
(click-to-pin) keeps its empty-state copy because an explicit pin is a
deliberate inspect action where confirming 'nothing here' is valid.

Tests:
- Inverted the two T24 hover tests to assert the tooltip does NOT render
  on unmodified pieces (b1 knight, e2 pawn on a vanilla solo game).
- Added a positive test: hover a modified pawn (HP +1 from a seeded
  profile) and assert the tooltip + at least one row are visible.

packages/chess/e2e/modifier-profiles.spec.ts

@@ -165,29 +165,85 @@ test.describe('Modifier Profiles — hover tooltip (T24)', () => {
     await page.waitForURL('**/game');
   });
 
-  test('hover piece shows modifier tooltip', async ({ page }) => {
+  test('hover unmodified piece does NOT show tooltip', async ({ page }) => {
-    // Hover white knight at b1 (standard opening position).
+    // Standard solo game — no modifier profile active, so b1 knight
+    // has no modifier facts. The tooltip only renders when the piece
+    // actually has modifiers; showing an empty card on every hover was
+    // noisy and unhelpful.
     await page.hover('[data-square="b1"]');
     await page.waitForTimeout(200);
 
-    // Tooltip must appear and identify the piece.
+    await expect(
+      page.locator('[data-testid="modifier-tooltip"]'),
+    ).toHaveCount(0);
+  });
+
+  test('hover unmodified pawn does NOT show tooltip', async ({ page }) => {
+    // Same invariant, second fixture piece. Guards the "no noise on
+    // unmodified pieces" behaviour in case a future change accidentally
+    // re-enables the always-render path.
+    await page.hover('[data-square="e2"]'); // white pawn
+    await page.waitForTimeout(200);
+
+    await expect(
+      page.locator('[data-testid="modifier-tooltip-row"]'),
+    ).toHaveCount(0);
+    await expect(
+      page.locator('[data-testid="modifier-tooltip"]'),
+    ).toHaveCount(0);
+  });
+
+  test('hover modified piece DOES show tooltip with rows', async ({ page }) => {
+    // Seed a profile that puts HP +1 on every white pawn, apply it to
+    // a fresh solo game via the lobby picker, then hover e2. The
+    // tooltip should render and surface the HP Bonus row.
+    await page.goto('/');
+    const LIBRARY_KEY = 'houserules:modifier-profiles:v1';
+    const entry = {
+      id: 't24-hover-positive',
+      name: 'Hover Positive',
+      profile: {
+        id: 't24-hover-positive',
+        name: 'Hover Positive',
+        description: '',
+        layoutId: 'classic',
+        perType: [
+          { kind: 'hp-bonus', pieceType: 'pawn', color: 'white', value: 1 },
+        ],
+        perInstance: [],
+        version: 1,
+        source: 'custom',
+      },
+      starred: false,
+      updatedAt: Date.now(),
+    };
+    await page.evaluate(
+      ({ key, e }) => localStorage.setItem(key, JSON.stringify([e])),
+      { key: LIBRARY_KEY, e: entry },
+    );
+    await page.reload();
+    const picker = page.getByTestId('profile-picker');
+    if (!(await picker.isVisible())) {
+      await page.locator('[data-action="open-rules-drawer"]').click();
+    }
+    await expect(picker).toBeVisible();
+    await picker.selectOption('t24-hover-positive');
+    await page.locator('[data-action="play-solo"]').click();
+    await page.waitForURL('**/game');
+
+    // Hover e2 — white pawn with HpBonus +1 from the profile.
+    await page.hover('[data-square="e2"]');
+    await page.waitForTimeout(200);
+
     await expect(
       page.locator('[data-testid="modifier-tooltip"]'),
     ).toBeVisible({ timeout: 1000 });
     await expect(
       page.locator('[data-testid="modifier-tooltip"]'),
-    ).toContainText(/knight/i);
+    ).toContainText(/pawn/i);
-  });
-
-  test('hover unmodified piece shows no modifier rows', async ({ page }) => {
-    // Standard solo game — no modifier profile active.
-    await page.hover('[data-square="e2"]'); // white pawn
-    await page.waitForTimeout(200);
-
-    // Tooltip appears (header always shows) but no modifier rows.
     await expect(
-      page.locator('[data-testid="modifier-tooltip-row"]'),
+      page.locator('[data-testid="modifier-tooltip-row"]').first(),
-    ).toHaveCount(0);
+    ).toBeVisible();
   });
 });
 

packages/chess/src/ui/ModifierTooltip.tsx

@@ -5,9 +5,10 @@
  * the engine session. Renders as an absolute-positioned card; the parent is
  * responsible for placement (positioned within a `relative` container).
  *
- * Source attribution is simplified for T24: any fact that matches a
+ * Renders nothing when the piece has no modifier facts — the tooltip exists
- * MODIFIER_REGISTRY attribute is shown without distinguishing per-type /
+ * to surface modifier state, and showing an empty card on every unmodified
- * per-instance / preset origin (T26 can refine this).
+ * piece is noisy. The pinned panel (click-to-pin) carries the "no modifiers"
+ * empty state instead, since an explicit pin is a deliberate inspect action.
  */
 import { asEntityId } from '@paratype/rete';
 import { MODIFIER_REGISTRY } from '../modifiers/index.js';
@@ -40,6 +41,11 @@ export function ModifierTooltip({ pieceId, engine }: Props) {
     return [{ id: descriptor.id, label: descriptor.label, description }];
   });
 
+  // No modifiers on this piece → don't render the tooltip at all. A card
+  // that just says "White Knight" with an empty-state line adds noise
+  // without carrying information the user can't already see on the board.
+  if (modifierRows.length === 0) return null;
+
   return (
     <div
       data-testid="modifier-tooltip"
@@ -63,11 +69,6 @@ export function ModifierTooltip({ pieceId, engine }: Props) {
           </span>
         </div>
       ))}
-
-      {/* Empty state */}
-      {modifierRows.length === 0 && (
-        <p className="text-[11px] text-neutral-400 italic">No active modifiers</p>
-      )}
     </div>
   );
 }