[WEB-5772] fix: theme switch flicker (#8428)

apps/web/core/lib/wrappers/store-wrapper.tsx

@@ -1,5 +1,5 @@
 import type { ReactNode } from "react";
-import { useEffect } from "react";
+import { useEffect, useRef } from "react";
 import { observer } from "mobx-react";
 import { useParams } from "next/navigation";
 import { useTheme } from "next-themes";
@@ -28,6 +28,13 @@ function StoreWrapper(props: TStoreWrapper) {
   const { data: userProfile } = useUserProfile();
   const { changeLanguage } = useTranslation();
 
+  // Track if we've initialized theme from server (one-time only)
+  const hasInitializedThemeRef = useRef(false);
+  // Track current user to reset on logout/login
+  const currentUserIdRef = useRef<string | undefined>(undefined);
+  // Track previous theme to detect transitions from custom theme
+  const previousThemeRef = useRef<string | undefined>(undefined);
+
   /**
    * Sidebar collapsed fetching from local storage
    */
@@ -38,25 +45,61 @@ function StoreWrapper(props: TStoreWrapper) {
   }, [sidebarCollapsed, setTheme, toggleSidebar]);
 
   /**
-   * Setting up the theme of the user by fetching it from profile
+   * Effect 1: Initial theme sync from server (one-time only)
+   *
+   * This effect runs ONCE per user session to load theme from server.
+   * After initial load, all theme changes are localStorage-driven (next-themes).
+   * This prevents a feedback loop where server updates trigger UI updates in a cycle.
+   */
+  useEffect(() => {
+    const userId = userProfile?.id;
+
+    // Reset initialization flag when user changes (logout/login)
+    // This handles both logout (userId becomes undefined) and login (userId changes)
+    if (userId !== currentUserIdRef.current) {
+      hasInitializedThemeRef.current = false;
+      previousThemeRef.current = undefined;
+      currentUserIdRef.current = userId;
+    }
+
+    // Only initialize theme from server on FIRST load for this user
+    if (!userProfile?.theme?.theme || hasInitializedThemeRef.current) {
+      return; // Skip if already initialized or no profile data
+    }
+
+    // Apply theme from server profile (one-time only)
+    setTheme(userProfile?.theme?.theme || "system");
+
+    // Mark as initialized - prevents future syncs from server
+    hasInitializedThemeRef.current = true;
+  }, [userProfile?.theme?.theme, setTheme]);
+
+  /**
+   * Effect 2: Custom theme CSS application (runs on every change)
+   *
+   * This effect applies or clears custom theme CSS variables whenever
+   * the theme changes. It runs independently of the initial sync effect.
    */
   useEffect(() => {
     if (!userProfile?.theme?.theme) return;
-    const currentTheme = userProfile?.theme?.theme || "system";
-    const theme = userProfile?.theme;
 
-    if (currentTheme) {
-      setTheme(currentTheme);
-      if (currentTheme === "custom") {
-        // New 2-color palette system
-        if (theme.primary && theme.background && theme.darkPalette !== undefined) {
-          applyCustomTheme(theme.primary, theme.background, theme.darkPalette ? "dark" : "light");
-        }
-      } else {
-        clearCustomTheme();
-      }
+    const currentTheme = userProfile?.theme?.theme;
+    const previousTheme = previousThemeRef.current;
+    const themeData = userProfile?.theme;
+
+    // Apply custom theme if current theme is custom
+    if (currentTheme === "custom" && themeData.primary && themeData.background && themeData.darkPalette !== undefined) {
+      applyCustomTheme(themeData.primary, themeData.background, themeData.darkPalette ? "dark" : "light");
     }
-  }, [userProfile?.theme, setTheme]);
+    // Clear custom theme CSS when switching away from custom
+    else if (previousTheme === "custom" && currentTheme !== "custom") {
+      clearCustomTheme();
+      // No reload needed - let CSS cascade handle it naturally
+    }
+
+    // Update previous theme for next comparison
+    previousThemeRef.current = currentTheme;
+  }, [userProfile?.theme]);
 
   useEffect(() => {
     if (!userProfile?.language) return;