diff --git a/TODO.md b/TODO.md
index 17ea36f..2ce199d 100644
--- a/TODO.md
+++ b/TODO.md
@@ -50,6 +50,19 @@
 - Test: `shared/tests/turn.characterization.test.js` 'addParticipant rejects
   duplicate id' — GREEN.
 
+### BUG-4: hide-player-HP breaks display view (preexisting)
+- Toggle "hide player HP" in admin → display view flips to "Game Session Paused".
+- Toggling back does NOT recover. Must re-activate encounter in encounters
+  panel to restore display.
+- Expected: hide-HP toggle updates one field on activeDisplay/status doc,
+  display stays live on current encounter.
+- Likely cause: toggle writes to wrong path, or clobbers activeCampaignId/
+  activeEncounterId with null (setDoc replace vs updateDoc patch).
+- Fix: use updateDoc (patch) not setDoc (replace); or include all existing
+  fields when writing.
+- Test: render App + DisplayView, toggle hide-HP, assert display still shows
+  encounter (not paused).
+
 ## Pipeline
 - [ ] Red test: dead participant still in turnOrderIds, turn still advances to them
 - [ ] Fix `shared/turn.js`: don't drop dead from turn order