fix(mobile): prevent pushState → popstate infinite loop in MobileApp URL sync

React error #185 (Maximum update depth exceeded) on mobile when navigating
to ?m=chat&a=... was caused by a render loop in the URL sync effect.

Root cause: when setRoute/setAgentId fires, the URL sync effect calls
pushState. Some mobile WebViews dispatch popstate synchronously as a
side-effect of pushState, which triggers the popstate handler → setRoute →
URL sync effect → pushState → popstate → ... infinite loop.

Fix: track what we just pushed in a prevPushedRef. The popstate handler
reads the current URL and skips the state update if the URL matches
prevPushedRef — that means the popstate was our own pushState's
side-effect, not a genuine user back-navigation.

Also tracks: react-hooks/exhaustive-props disabled on the URL sync
effect since prevPushedRef is updated inside the effect body (intentional
self-mutation pattern for cross-effect communication).

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

canvas/src/components/mobile/MobileApp.tsx

@@ -6,7 +6,7 @@
 // instantiated when MobileApp is active, so no React Flow + heavy
 // chrome cost on phones.
 
-import { useEffect, useMemo, useState } from "react";
+import { useEffect, useMemo, useRef, useState } from "react";
 
 import { useTheme } from "@/lib/theme-provider";
 
@@ -90,20 +90,30 @@ export function MobileApp() {
   // the URL is already what we'd produce — that handles the initial
   // mount (we read FROM the URL) and prevents duplicate history entries
   // when popstate restores state we just pushed.
+  // Track what we pushed so the popstate handler can distinguish
+  // "browser back to our own push" (skip) from "browser back to
+  // user navigation" (update state).
+  const prevPushedRef = useRef<{ route: Route; agentId: string | null }>({ route: route as Route, agentId });
   useEffect(() => {
     if (typeof window === "undefined") return;
     const current = readRouteFromUrl();
     if (current.route === route && current.agentId === agentId) return;
     const url = buildRouteUrl(route, agentId);
+    prevPushedRef.current = { route, agentId };
     window.history.pushState({ route, agentId }, "", url);
-  }, [route, agentId]);
+  }, [route, agentId]); // eslint-disable-line react-hooks/exhaustive-props
 
   // Sync URL → route state on browser back/forward. The popstate event
   // fires AFTER the URL has changed, so re-reading is correct.
+  // Skip: if the URL now reflects the state we just pushed (popstate was
+  // fired synchronously by our own pushState on some mobile browsers).
   useEffect(() => {
     if (typeof window === "undefined") return;
     const onPop = () => {
       const next = readRouteFromUrl();
+      // If the URL reflects what we just pushed, skip — this popstate
+      // was a side-effect of our own pushState, not user navigation.
+      if (next.route === prevPushedRef.current.route && next.agentId === prevPushedRef.current.agentId) return;
       setRoute(next.route);
       setAgentId(next.agentId);
     };