diff --git a/TODO.md b/TODO.md
index 54e6164..2e4eaa7 100644
--- a/TODO.md
+++ b/TODO.md
@@ -90,7 +90,7 @@ REWORK_PLAN.md.
 
 ### bug-3 was a halucination has been removed
 
-### BUG-4: hide-player-HP breaks display view (preexisting)
+### BUG-4: hide-player-HP breaks display view (preexisting) --- PROD FIXED, TEST RED (mock bug)
 - **Broader than hide-HP**: ALL 5 `storage.setDoc(getPath.activeDisplay(), ...)` calls
   use `{merge:true}` which is IGNORED (setDoc = replace per contract).
   Each write clobbers other fields on activeDisplay/status doc.
@@ -108,6 +108,13 @@ REWORK_PLAN.md.
   activeEncounterId with null (setDoc replace vs updateDoc patch).
 - Fix: use updateDoc (patch) not setDoc (replace); or include all existing
   fields when writing.
+- Status update (2026-07): all 5 sites now use `{merge:true}`. Real firebase
+  adapter honors merge → production works. BUT jsdom test still RED because
+  `src/__mocks__/firebase/firestore.js` setDoc records call, IGNORES opts
+  (no actual merge). Mock must simulate firebase merge semantics for test
+  to pass. Fix = mock setDoc: if opts.merge, MOCK_DB.merge(path,data) else
+  replace. OR change App.js setDoc(merge) → updateDoc (cleaner, ws adapter
+  uses PATCH). Decide which.
 - Test: render App + DisplayView, toggle hide-HP, assert display still shows
   encounter (not paused).
 
diff --git a/src/App.js b/src/App.js
index 97cd7a8..d296f2a 100644
--- a/src/App.js
+++ b/src/App.js
@@ -1662,7 +1662,7 @@ function InitiativeControls({ campaignId, encounter, encounterPath }) {
   const handleToggleHidePlayerHp = async () => {
     if (!db) return;
     try {
-      await storage.setDoc(getPath.activeDisplay(), { hidePlayerHp: !hidePlayerHp }, { merge: true });
+      await storage.updateDoc(getPath.activeDisplay(), { hidePlayerHp: !hidePlayerHp });
     } catch (err) {
       console.error("Error toggling hidePlayerHp:", err);
     }
@@ -1695,10 +1695,10 @@ function InitiativeControls({ campaignId, encounter, encounterPath }) {
         turnOrderIds: sortedParticipants.map(p => p.id)
       });
 
-      await storage.setDoc(getPath.activeDisplay(), {
+      await storage.updateDoc(getPath.activeDisplay(), {
         activeCampaignId: campaignId,
         activeEncounterId: encounter.id
-      }, { merge: true });
+      });
 
       logAction(`Combat started: "${encounter.name}" — ${sortedParticipants[0].name}'s turn (Round 1)`, { encounterName: encounter.name }, {
         encounterPath,
@@ -1826,10 +1826,10 @@ function InitiativeControls({ campaignId, encounter, encounterPath }) {
         turnOrderIds: []
       });
 
-      await storage.setDoc(getPath.activeDisplay(), {
+      await storage.updateDoc(getPath.activeDisplay(), {
         activeCampaignId: null,
         activeEncounterId: null
-      }, { merge: true });
+      });
 
       logAction(`Combat ended: "${encounter.name}"`, { encounterName: encounter.name }, {
         encounterPath,
@@ -2044,15 +2044,15 @@ function EncounterManager({ campaignId, initialActiveEncounterId, campaignCharac
       const currentActiveEncounter = activeDisplayInfo?.activeEncounterId;
 
       if (currentActiveCampaign === campaignId && currentActiveEncounter === encounterId) {
-        await storage.setDoc(getPath.activeDisplay(), {
+        await storage.updateDoc(getPath.activeDisplay(), {
           activeCampaignId: null,
           activeEncounterId: null,
-        }, { merge: true });
+        });
       } else {
-        await storage.setDoc(getPath.activeDisplay(), {
+        await storage.updateDoc(getPath.activeDisplay(), {
           activeCampaignId: campaignId,
           activeEncounterId: encounterId,
-        }, { merge: true });
+        });
       }
     } catch (err) {
       console.error("Error toggling Player Display:", err);
diff --git a/src/tests/Combat.characterization.test.js b/src/tests/Combat.characterization.test.js
index 9a4a852..618762d 100644
--- a/src/tests/Combat.characterization.test.js
+++ b/src/tests/Combat.characterization.test.js
@@ -41,7 +41,7 @@ describe('Combat -> Firebase', () => {
   test('startEncounter: also sets activeDisplay to this encounter', async () => {
     await setupWithMonsters();
     await startCombatViaUI();
-    const adCalls = findCallActiveDisplay('setDoc');
+    const adCalls = findCallActiveDisplay('updateDoc');
     const last = adCalls[adCalls.length - 1];
     expect(last.data.activeCampaignId).toBeTruthy();
     expect(last.data.activeEncounterId).toBeTruthy();
@@ -111,26 +111,26 @@ describe('Combat -> Firebase', () => {
     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 adCalls = findCallActiveDisplay('updateDoc');
       const last = adCalls[adCalls.length - 1];
       return last && last.data.activeCampaignId === null;
     });
-    const adCalls = findCallActiveDisplay('setDoc');
+    const adCalls = findCallActiveDisplay('updateDoc');
     const last = adCalls[adCalls.length - 1];
     expect(last.data).toMatchObject({ activeCampaignId: null, activeEncounterId: null });
   });
 
-  test('toggleHidePlayerHp: setDoc merge on activeDisplay/status', async () => {
+  test('toggleHidePlayerHp: updateDoc patch on activeDisplay/status', async () => {
     await setupWithMonsters();
     await startCombatViaUI();
     const switchBtn = screen.getByRole('switch');
     fireEvent.click(switchBtn);
     await waitFor(() => {
-      const adCalls = findCallActiveDisplay('setDoc');
+      const adCalls = findCallActiveDisplay('updateDoc');
       const last = adCalls[adCalls.length - 1];
       return last && 'hidePlayerHp' in last.data;
     });
-    const adCalls = findCallActiveDisplay('setDoc');
+    const adCalls = findCallActiveDisplay('updateDoc');
     const last = adCalls[adCalls.length - 1];
     expect(last.data).toHaveProperty('hidePlayerHp');
   });
diff --git a/src/tests/Encounter.characterization.test.js b/src/tests/Encounter.characterization.test.js
index 4a9e959..acaab44 100644
--- a/src/tests/Encounter.characterization.test.js
+++ b/src/tests/Encounter.characterization.test.js
@@ -42,7 +42,7 @@ describe('Encounter -> Firebase', () => {
     expect(call.path).toMatch(/campaigns\/[^/]+\/encounters\//);
   });
 
-  test('togglePlayerDisplay: setDoc merge on activeDisplay/status', async () => {
+  test('togglePlayerDisplay: updateDoc patch on activeDisplay/status', async () => {
     await setupCampaignAndEncounter('Camp D', 'Enc D');
     await selectEncounterByName('Enc D');
 
@@ -50,33 +50,33 @@ describe('Encounter -> Firebase', () => {
     const eyeBtn = await screen.findByTitle('Activate for Player Display');
     fireEvent.click(eyeBtn);
 
-    await waitFor(() => findCall('setDoc', 'activeDisplay/status'));
-    const call = findCall('setDoc', 'activeDisplay/status');
-    // activeDisplay/status setDoc is called with merge option in App
+    await waitFor(() => findCall('updateDoc', 'activeDisplay/status'));
+    const call = findCall('updateDoc', 'activeDisplay/status');
+    // BUG-4 fix: updateDoc patch, not setDoc replace (was clobbering fields)
     expect(call.data).toMatchObject({
       activeCampaignId: expect.any(String),
       activeEncounterId: expect.any(String),
     });
   });
 
-  test('togglePlayerDisplay off: setDoc nulls active ids', async () => {
+  test('togglePlayerDisplay off: updateDoc nulls active ids', async () => {
     await setupCampaignAndEncounter('Camp O', 'Enc O');
     await selectEncounterByName('Enc O');
 
     // turn ON
     const onBtn = await screen.findByTitle('Activate for Player Display');
     fireEvent.click(onBtn);
-    await waitFor(() => findCall('setDoc', 'activeDisplay/status'));
+    await waitFor(() => findCall('updateDoc', 'activeDisplay/status'));
 
     // turn OFF
     const offBtn = await screen.findByTitle('Deactivate for Player Display');
     fireEvent.click(offBtn);
     await waitFor(() => {
-      const calls = findCalls('setDoc', 'activeDisplay/status');
+      const calls = findCalls('updateDoc', 'activeDisplay/status');
       const last = calls[calls.length - 1];
       return last.data.activeCampaignId === null;
     });
-    const calls = findCalls('setDoc', 'activeDisplay/status');
+    const calls = findCalls('updateDoc', 'activeDisplay/status');
     const last = calls[calls.length - 1];
     expect(last.data).toMatchObject({ activeCampaignId: null, activeEncounterId: null });
   });
@@ -103,7 +103,7 @@ describe('Encounter -> Firebase', () => {
     // activate display first
     const onBtn = await screen.findByTitle('Activate for Player Display');
     fireEvent.click(onBtn);
-    await waitFor(() => findCall('setDoc', 'activeDisplay/status'));
+    await waitFor(() => findCall('updateDoc', 'activeDisplay/status'));
 
     // delete the active encounter
     const trashBtn = screen.getAllByTitle('Delete Encounter')[0];
diff --git a/src/tests/HideHpToggle.test.js b/src/tests/HideHpToggle.test.js
index 0e44614..4dee45e 100644
--- a/src/tests/HideHpToggle.test.js
+++ b/src/tests/HideHpToggle.test.js
@@ -50,14 +50,14 @@ describe('BUG-4: hide-player-HP toggle preserves activeDisplay', () => {
 
     await waitFor(() => {
       const writes = getAdapterCalls().filter(
-        c => c.fn === 'setDoc' && c.path.includes('activeDisplay/status')
+        c => c.fn === 'updateDoc' && c.path.includes('activeDisplay/status')
       );
       expect(writes.length).toBeGreaterThan(0);
       const last = writes[writes.length - 1];
-      // data written must include activeCampaignId AND activeEncounterId
-      // BUG: writes only {hidePlayerHp:true}, clobbering them.
-      expect(last.data.activeCampaignId).toBe('c1');
-      expect(last.data.activeEncounterId).toBe('e1');
+      // patch must NOT clobber activeCampaignId/activeEncounterId.
+      // BUG: setDoc replace writes only {hidePlayerHp:true} → clobbers.
+      // Fix: updateDoc patch — other fields untouched.
+      expect(last.patch.hidePlayerHp).toBe(true);
     }, { timeout: 3000 });
   });
 });