From 0f78e1ece237a7b924ebf6d4687819cf18d7462d Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Philippe=20Rivi=C3=A8re?= <fil@rezo.net>
Date: Fri, 3 Jul 2026 23:45:11 +0200
Subject: [PATCH] a stale observer must not update the display (using a WeakMap
 of the current observer for a given display)

closes #174
---
 src/runtime/display.ts | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/src/runtime/display.ts b/src/runtime/display.ts
index 97f9b25..5763e39 100644
--- a/src/runtime/display.ts
+++ b/src/runtime/display.ts
@@ -49,17 +49,21 @@ export function clear(state: DisplayState): void {
   while (state.root.lastChild) state.root.lastChild.remove();
 }
 
+const observers = new WeakMap<DisplayState, unknown>();
+
 export function observe(state: DisplayState, {autodisplay, assets, output}: Definition) {
-  return {
+  const observer = {
     _error: false,
     _node: state.root, // _node for visibility promise
     pending() {
+      if (observers.get(state) !== this) return; // stale, e.g., after the cell is redefined
       if (this._error) {
         this._error = false;
         clear(state);
       }
     },
     fulfilled(value: unknown) {
+      if (observers.get(state) !== this) return;
       if (autodisplay) {
         if (assets && value instanceof Element) mapAssets(value, assets);
         clear(state);
@@ -69,10 +73,13 @@ export function observe(state: DisplayState, {autodisplay, assets, output}: Defi
       }
     },
     rejected(error: unknown) {
+      if (observers.get(state) !== this) return;
       console.error(error);
       this._error = true;
       clear(state);
       displayError(state, error, output);
     }
   };
+  observers.set(state, observer);
+  return observer;
 }