diff --git a/.serena/memories/frontend/dashboard-viewer-subtleties.md b/.serena/memories/frontend/dashboard-viewer-subtleties.md
index 975d410dea..f4cfd45cc9 100644
--- a/.serena/memories/frontend/dashboard-viewer-subtleties.md
+++ b/.serena/memories/frontend/dashboard-viewer-subtleties.md
@@ -13,4 +13,4 @@
 - Viewer bundle fetch sends the full supported feature set because anonymous shared viewers may not know team-enabled features.
 - View-only bundles can contain pointer values in `:pages-index` and file data. Viewer resolves those fragments with `:get-file-fragment` before storing the bundle.
 - `bundle-fetched` indexes pages and precomputes viewer frames/all-frames, stores libraries/users/thumbnails/permissions under `:viewer`, then navigates to frame id, query index, or auto-selected frame.
-- Viewer zoom and interaction mode changes update both `:viewer-local` and the `:viewer` route query params.
\ No newline at end of file
+- Viewer zoom and interaction mode changes update both `:viewer-local` and the `:viewer` route query params. `update-zoom-querystring` guards with a query-param comparison (`not= current expected`) and uses `::rt/replace`; without that guard the load sequence (`bundle-fetched` → `zoom-to-fill` → `update-zoom-querystring` → `rt/nav`) re-runs on every navigation and crashes with React "maximum update depth exceeded" (2.18.0-RC5 regression, fixed in `31b73460c3`; regression test: `bundle-fetched-with-zoom-fill-url-does-not-navigate`).
\ No newline at end of file
diff --git a/frontend/test/frontend_tests/data/viewer_test.cljs b/frontend/test/frontend_tests/data/viewer_test.cljs
index 482210b24d..5c3b0ca128 100644
--- a/frontend/test/frontend_tests/data/viewer_test.cljs
+++ b/frontend/test/frontend_tests/data/viewer_test.cljs
@@ -122,3 +122,32 @@
           {:keys [params]} (some-> (first events) deref)]
       (t/is (= 1 (count events)))
       (t/is (not (contains? params :zoom))))))
+
+(t/deftest bundle-fetched-with-zoom-fill-url-does-not-navigate
+  ;; Regression test for the React "maximum update depth exceeded"
+  ;; error (2.18.0-RC5): loading the viewer with a URL that already
+  ;; contains `zoom=fill` re-entered the cycle zoom-to-fill →
+  ;; update-zoom-querystring → nav → navigated → zoom-to-fill…,
+  ;; because update-zoom-querystring navigated unconditionally.
+  ;; At HEAD the guard breaks the cycle, so a bundle fetch must not
+  ;; emit any navigation.
+  (let [state (-> (base-state {:frames [{:selrect {:width 100 :height 100}}]
+                               :index  0})
+                  (assoc-in [:route :query-params :zoom] "fill"))]
+    (let [events (watch-events (dv/bundle-fetched
+                                {:file {:id page-id
+                                        :data {:pages [page-id]
+                                               :pages-index {page-id {:objects {}}}}}
+                                 :project {}
+                                 :team {:features []}
+                                 :share-links []
+                                 :libraries []
+                                 :users []
+                                 :permissions {}
+                                 :thumbnails {}})
+                               state)
+          ;; `update-page-position-data` and `go-to-frame-auto` are part
+          ;; of the normal init sequence; the navigation events are the
+          ;; ones that can restart the zoom cycle.
+          navigations (filter #(= :app.main.router/navigate (ptk/type %)) events)]
+      (t/is (empty? navigations)))))