fix(sidebar): suppress collapse->expand transition flash on fresh load

apps/sim/app/_styles/globals.css

@@ -110,6 +110,18 @@ html.sidebar-resizing .sidebar-shell-inner {
   transition: none !important;
 }
 
+/* Suppress sidebar transitions during the initial hydration window. The
+   pre-paint script sets the correct --sidebar-width, but store rehydration
+   re-applies it a tick later; without this guard that re-apply animates the
+   rail, reading as a collapse -> expand flash on a fresh page load. Removed
+   after the first paint (see workspace-chrome.tsx) so user-driven toggles and
+   the fullscreen slide still animate. */
+html.sidebar-booting .sidebar-container,
+html.sidebar-booting .sidebar-shell-outer,
+html.sidebar-booting .sidebar-shell-inner {
+  transition: none !important;
+}
+
 .panel-container {
   width: var(--panel-width);
 }

apps/sim/app/workspace/[workspaceId]/components/workspace-chrome/workspace-chrome.tsx

@@ -1,6 +1,6 @@
 'use client'
 
-import { useEffect, useLayoutEffect } from 'react'
+import { useEffect, useLayoutEffect, useRef } from 'react'
 import { cn } from '@sim/emcn'
 import { usePathname } from 'next/navigation'
 import { Sidebar } from '@/app/workspace/[workspaceId]/w/components/sidebar/sidebar'
@@ -47,6 +47,8 @@ export function WorkspaceChrome({
   children,
   initialSidebarCollapsed = false,
 }: WorkspaceChromeProps) {
+  const rafRef = useRef(0)
+
   const pathname = usePathname()
   const isFullscreen = isFullscreenPath(pathname)
 
@@ -66,6 +68,29 @@ export function WorkspaceChrome({
    */
   const isCollapsed = hasHydrated ? storeIsCollapsed : initialSidebarCollapsed
 
+  /**
+   * Suppresses sidebar transitions across the initial hydration window. The
+   * pre-paint script already set the correct `--sidebar-width`, but the store
+   * rehydration below re-applies it a tick later; without this guard that
+   * re-apply animates the rail, reading as a collapse -> expand flash on a
+   * fresh load. Applied before the rehydrate effect so the class is in place
+   * ahead of the width mutation, then lifted after the first paint so
+   * user-driven collapse toggles and the fullscreen slide still animate.
+   */
+  useLayoutEffect(() => {
+    const root = document.documentElement
+    root.classList.add('sidebar-booting')
+    const raf1 = requestAnimationFrame(() => {
+      const raf2 = requestAnimationFrame(() => root.classList.remove('sidebar-booting'))
+      rafRef.current = raf2
+    })
+    rafRef.current = raf1
+    return () => {
+      cancelAnimationFrame(rafRef.current)
+      root.classList.remove('sidebar-booting')
+    }
+  }, [])
+
   // Hydrate the persisted width before paint (collapse comes from the cookie/prop).
   useLayoutEffect(() => {
     void useSidebarStore.persist.rehydrate()