From d851f82678cf77c61c269762985bf8910c621803 Mon Sep 17 00:00:00 2001
From: Alonso Torres <alonso.torres@kaleidos.net>
Date: Thu, 1 Oct 2026 09:21:05 +0200
Subject: [PATCH] :bug: Fix crashing happening on hot reloads (#12005)

---
 frontend/src/app/main.cljs            |  9 ++++++++-
 frontend/src/app/render_wasm/api.cljs | 15 +++++++++++----
 2 files changed, 19 insertions(+), 5 deletions(-)

diff --git a/frontend/src/app/main.cljs b/frontend/src/app/main.cljs
index b6fbfb3ca5..c7d87ba8ba 100644
--- a/frontend/src/app/main.cljs
+++ b/frontend/src/app/main.cljs
@@ -54,8 +54,12 @@
   (let [el (dom/get-element "app")]
     (mf/create-root el)))
 
+;; Root component of the last render; a hot reload of `app.main.ui` replaces it
+(defonce ^:private rendered-app (atom nil))
+
 (defn init-ui
   []
+  (reset! rendered-app ui/app)
   (mf/render! app-root (mf/element ui/app)))
 
 (defn- initialize-rasterizer
@@ -146,7 +150,10 @@
    (init-ui)))
 
 (defn ^:dev/after-load after-load
+  "Rerender the UI after a hot reload. When the reload replaced the root
+  component, unmount the whole tree first so its cleanups run before
+  the new tree mounts."
   []
-  (reinit))
+  (reinit (not (identical? @rendered-app ui/app))))
 
 (set! (.-stackTraceLimit js/Error) 50)
diff --git a/frontend/src/app/render_wasm/api.cljs b/frontend/src/app/render_wasm/api.cljs
index 0c5a4c3d0b..78623aa43e 100644
--- a/frontend/src/app/render_wasm/api.cljs
+++ b/frontend/src/app/render_wasm/api.cljs
@@ -2492,6 +2492,13 @@
                               :cause cause)
                    nil)))))
 
+;; Stable listener identities, so `removeEventListener` matches across hot reloads
+(defonce ^:private webgl-context-lost-listener
+  (fn [event] (on-webgl-context-lost event)))
+
+(defonce ^:private webgl-context-restored-listener
+  (fn [event] (on-webgl-context-restored event)))
+
 (defn init-canvas-context
   [canvas]
   (if-not (wasm/module-ready?)
@@ -2536,8 +2543,8 @@
           ;; Add event listeners for WebGL context lost
           (set! wasm/canvas canvas)
           (when can-listen?
-            (.addEventListener canvas "webglcontextlost" on-webgl-context-lost)
-            (.addEventListener canvas "webglcontextrestored" on-webgl-context-restored))
+            (.addEventListener canvas "webglcontextlost" webgl-context-lost-listener)
+            (.addEventListener canvas "webglcontextrestored" webgl-context-restored-listener))
           (start-canvas-snapshot-listener!)
           (reset! wasm/context-lost? false)
           (set! wasm/context-initialized? true)
@@ -2571,8 +2578,8 @@
 
      ;; Remove listener before losing/deleting context.
      (when wasm/canvas
-       (.removeEventListener wasm/canvas "webglcontextlost" on-webgl-context-lost)
-       (.removeEventListener wasm/canvas "webglcontextrestored" on-webgl-context-restored))
+       (.removeEventListener wasm/canvas "webglcontextlost" webgl-context-lost-listener)
+       (.removeEventListener wasm/canvas "webglcontextrestored" webgl-context-restored-listener))
      (stop-canvas-snapshot-listener!)
 
      (when (wasm/module-ready?)