fix: guard player display from cross-encounter conflicts

- Start encounter only claims display if slot empty or already ours
  (prevents stealing display from another live encounter)
- End encounter only clears display if THIS encounter is the one showing
  (prevents killing display for a different live encounter)
- Use unwrapped activeDisplayData (not snapshot wrapper activeDisplayInfo)
- Tests: 4 display guard cases (claim empty, no-steal busy, clear own,
  no-clear other)

src/App.js

@@ -2165,7 +2165,13 @@ function InitiativeControls({ campaignId, encounter, encounterPath }) {
 
     try {
       await startEncounter(encounter, buildCtx(encounterPath));
-      await storage.setDoc(getPath.activeDisplay(), { activeCampaignId: campaignId, activeEncounterId: encounter.id }, { merge: true });
+      // Only claim player display if slot is empty or already showing this encounter.
+      // Don't steal display from another live encounter.
+      const displayEmpty = !activeDisplayData || !activeDisplayData.activeEncounterId;
+      const displayIsOurs = activeDisplayData && activeDisplayData.activeCampaignId === campaignId && activeDisplayData.activeEncounterId === encounter.id;
+      if (displayEmpty || displayIsOurs) {
+        await storage.setDoc(getPath.activeDisplay(), { activeCampaignId: campaignId, activeEncounterId: encounter.id }, { merge: true });
+      }
     } catch (err) {
       showToast(err.message || "Failed to start encounter. Please try again.");    }
   };
@@ -2207,7 +2213,13 @@ function InitiativeControls({ campaignId, encounter, encounterPath }) {
         } catch {}
       }
       await endEncounter(encounter, ctx);
-      await storage.setDoc(getPath.activeDisplay(), { activeCampaignId: null, activeEncounterId: null }, { merge: true });
+      // Only clear display if THIS encounter is the one showing.
+      const displayIsThis = activeDisplayData &&
+        activeDisplayData.activeCampaignId === campaignId &&
+        activeDisplayData.activeEncounterId === encounter.id;
+      if (displayIsThis) {
+        await storage.setDoc(getPath.activeDisplay(), { activeCampaignId: null, activeEncounterId: null }, { merge: true });
+      }
     } catch (err) {
       showToast("Failed to end encounter. Please try again.");    }
 

src/tests/DisplayGuard.test.js

@@ -0,0 +1,94 @@
+// Display guard: starting encounter does NOT steal display from another live encounter.
+// Ending encounter does NOT clear display if different encounter showing.
+
+import React from 'react';
+import { screen, fireEvent, waitFor } from '@testing-library/react';
+import '@testing-library/jest-dom';
+import { getCalls, MOCK_DB } from '../__mocks__/firebase/_mock-db';
+import { setupReady, addMonsterViaUI, startCombatViaUI } from './testHelpers';
+
+const DISPLAY_PATH = 'artifacts/ttrpg-initiative-tracker-default/public/data/activeDisplay/status';
+
+function findCallActiveDisplay(fn) {
+  return getCalls().filter(c => c.fn === fn && c.path.includes('activeDisplay/status'));
+}
+
+describe('Display guard', () => {
+  test('startEncounter: claims display when slot empty', async () => {
+    await setupReady('GuardCamp1', 'GuardEnc1');
+    await addMonsterViaUI('Goblin', 10, 5);
+    await startCombatViaUI();
+    const adCalls = findCallActiveDisplay('setDoc');
+    const last = adCalls[adCalls.length - 1];
+    expect(last).toBeTruthy();
+    expect(last.data.activeEncounterId).toBeTruthy();
+  });
+
+  test('startEncounter: does NOT steal display from another live encounter', async () => {
+    // Pre-seed display as busy with a DIFFERENT encounter
+    MOCK_DB.set(DISPLAY_PATH, {
+      activeCampaignId: 'other-camp',
+      activeEncounterId: 'other-enc',
+    });
+
+    await setupReady('GuardCamp2', 'GuardEnc2');
+    await addMonsterViaUI('Orc', 15, 8);
+    await startCombatViaUI();
+
+    // Combat started on encounter doc
+    const encCalls = getCalls().filter(c => c.fn === 'updateDoc' && c.path.includes('/encounters/'));
+    expect(encCalls.some(c => c.data.isStarted === true)).toBe(true);
+
+    // But display setDoc should NOT have been called to claim this encounter
+    const adClaims = findCallActiveDisplay('setDoc').filter(
+      c => c.data && c.data.activeEncounterId && c.data.activeEncounterId !== 'other-enc'
+    );
+    expect(adClaims).toHaveLength(0);
+  });
+
+  test('endEncounter: clears display when ending the displayed encounter', async () => {
+    await setupReady('GuardCamp3', 'GuardEnc3');
+    await addMonsterViaUI('Wolf', 8, 3);
+    await startCombatViaUI();
+
+    fireEvent.click(screen.getByRole('button', { name: /End Combat/i }));
+    fireEvent.click(await screen.findByRole('button', { name: /Confirm/i }));
+
+    await waitFor(() => {
+      const adCalls = findCallActiveDisplay('setDoc');
+      const last = adCalls[adCalls.length - 1];
+      return last && last.data && last.data.activeCampaignId === null;
+    });
+    const adCalls = findCallActiveDisplay('setDoc');
+    const last = adCalls[adCalls.length - 1];
+    expect(last.data).toMatchObject({ activeCampaignId: null, activeEncounterId: null });
+  });
+
+  test('endEncounter: does NOT clear display when different encounter is live', async () => {
+    await setupReady('GuardCamp4', 'GuardEnc4');
+    await addMonsterViaUI('Bat', 4, 2);
+    await startCombatViaUI();
+
+    // Now hijack display to a different encounter (simulating eyeball toggle elsewhere)
+    MOCK_DB.set(DISPLAY_PATH, {
+      activeCampaignId: 'different-camp',
+      activeEncounterId: 'different-enc',
+    });
+
+    fireEvent.click(screen.getByRole('button', { name: /End Combat/i }));
+    fireEvent.click(await screen.findByRole('button', { name: /Confirm/i }));
+
+    // Wait for encounter to end
+    await waitFor(() => {
+      const encCalls = getCalls().filter(c => c.fn === 'updateDoc' && c.path.includes('/encounters/'));
+      const last = encCalls[encCalls.length - 1];
+      return last && last.data.isStarted === false;
+    });
+
+    // Display should NOT have been cleared
+    const clearCalls = findCallActiveDisplay('setDoc').filter(
+      c => c.data && c.data.activeCampaignId === null
+    );
+    expect(clearCalls).toHaveLength(0);
+  });
+});