diff --git a/app/[locale]/page.tsx b/app/[locale]/page.tsx
index a700de1c..31253d06 100644
--- a/app/[locale]/page.tsx
+++ b/app/[locale]/page.tsx
@@ -2121,25 +2121,44 @@ export default function Home() {
           <InlineAppView apps={loadedApps} activeAppId={inlineApp!.id} onClose={closeInlineApp} className="flex-1" />
         )}
 
-        {/* Mobile/Tablet Sidebar Overlay Backdrop */}
+        {/* Mobile/Tablet Sidebar Overlay Backdrop.
+            When embedded in a Pro pane the viewport is desktop-wide, so the
+            `lg:hidden` viewport-variant alone wouldn't gate this overlay;
+            scope to the pane via `absolute` so the backdrop stays inside
+            the pane instead of covering the whole window. */}
         {(isMobile || isTablet) && sidebarOpen && !inlineApp && (
           <div
-            className="fixed inset-0 bg-black/50 z-40 lg:hidden"
+            className={cn(
+              "inset-0 bg-black/50 z-40",
+              isEmbedded ? "absolute" : "fixed lg:hidden"
+            )}
             onClick={() => setSidebarOpen(false)}
           />
         )}
 
-        {/* Sidebar - overlay on mobile/tablet, fixed on desktop */}
+        {/* Sidebar - overlay on mobile/tablet, in-flow on desktop.
+            When embedded, overlay-mode is driven by pane-aware JS rather
+            than viewport-variants (which still see the full window). */}
         <div
           className={cn(
             "flex-shrink-0 h-full z-50",
             !isResizing && "transition-[width] duration-300",
-            // Mobile/Tablet: fixed overlay
-            "max-lg:fixed max-lg:inset-y-0 max-lg:left-0 max-lg:w-72 max-lg:pt-[env(safe-area-inset-top)]",
-            "max-lg:transform max-lg:transition-transform max-lg:duration-300 max-lg:ease-in-out",
-            !sidebarOpen && "max-lg:-translate-x-full",
-            // Desktop: normal flow
-            "lg:relative lg:translate-x-0",
+            isEmbedded
+              ? (isMobile || isTablet
+                  ? cn(
+                      "absolute inset-y-0 left-0 w-72 pt-[env(safe-area-inset-top)]",
+                      "transform transition-transform duration-300 ease-in-out",
+                      !sidebarOpen && "-translate-x-full"
+                    )
+                  : "relative translate-x-0")
+              : cn(
+                  // Mobile/Tablet: fixed overlay
+                  "max-lg:fixed max-lg:inset-y-0 max-lg:left-0 max-lg:w-72 max-lg:pt-[env(safe-area-inset-top)]",
+                  "max-lg:transform max-lg:transition-transform max-lg:duration-300 max-lg:ease-in-out",
+                  !sidebarOpen && "max-lg:-translate-x-full",
+                  // Desktop: normal flow
+                  "lg:relative lg:translate-x-0"
+                ),
             inlineApp && "hidden"
           )}
           style={!isMobile && !isTablet ? { width: sidebarCollapsed ? 64 : sidebarWidth } : undefined}
diff --git a/app/[locale]/pro/page.tsx b/app/[locale]/pro/page.tsx
index cab5a8fe..14d2bb79 100644
--- a/app/[locale]/pro/page.tsx
+++ b/app/[locale]/pro/page.tsx
@@ -1,6 +1,6 @@
 "use client";
 
-import { useEffect, useMemo, useState, type ComponentType, type DragEvent } from "react";
+import { useEffect, useMemo, useRef, useState, type ComponentType, type DragEvent } from "react";
 import { useTranslations } from "next-intl";
 import { NavigationRail } from "@/components/layout/navigation-rail";
 import { KeyboardShortcutsModal } from "@/components/keyboard-shortcuts-modal";
@@ -11,6 +11,7 @@ import { useAuthStore, redirectToLogin } from "@/stores/auth-store";
 import { useEmailStore } from "@/stores/email-store";
 import { useDeviceDetection } from "@/hooks/use-media-query";
 import { EmbeddedContext } from "@/hooks/use-is-embedded";
+import { PaneSizeContext } from "@/hooks/use-pane-size";
 import { ProTabBar, PRO_TAB_DRAG_MIME } from "@/components/pro/pro-tab-bar";
 import { useProTabStore, type ProTab, type ProTabKind, type ProPaneId } from "@/stores/pro-tab-store";
 import { cn } from "@/lib/utils";
@@ -54,25 +55,50 @@ interface PaneProps {
 }
 
 function Pane({ paneId, tabs, activeTabId, loadedTabIds, onPaneFocus, isFocused }: PaneProps) {
+  const paneRef = useRef<HTMLDivElement | null>(null);
+  // Measured pane width, published to children via PaneSizeContext so that
+  // useDeviceDetection / useIsMobile / etc. branch on pane width — not full
+  // viewport — and inner pages collapse to their mobile/tablet layouts when
+  // the pane is narrow.
+  const [paneWidth, setPaneWidth] = useState<number | null>(null);
+
+  useEffect(() => {
+    const el = paneRef.current;
+    if (!el || typeof ResizeObserver === "undefined") return;
+    const initialRect = el.getBoundingClientRect();
+    if (initialRect.width > 0) setPaneWidth(initialRect.width);
+    const ro = new ResizeObserver((entries) => {
+      const entry = entries[0];
+      if (!entry) return;
+      const w = entry.contentRect.width;
+      setPaneWidth((prev) => (prev !== null && Math.abs(prev - w) < 0.5 ? prev : w));
+    });
+    ro.observe(el);
+    return () => ro.disconnect();
+  }, []);
+
   return (
     <div
+      ref={paneRef}
       className="relative flex flex-1 flex-col overflow-hidden min-w-0 min-h-0"
       onMouseDownCapture={() => { if (!isFocused) onPaneFocus(paneId); }}
     >
-      {tabs
-        .filter((tab) => loadedTabIds.includes(tab.id))
-        .map((tab) => {
-          const isActive = tab.id === activeTabId;
-          return (
-            <div
-              key={tab.id}
-              className={cn("absolute inset-0 overflow-hidden", !isActive && "hidden")}
-              aria-hidden={!isActive}
-            >
-              {renderTabBody(tab)}
-            </div>
-          );
-        })}
+      <PaneSizeContext.Provider value={paneWidth}>
+        {tabs
+          .filter((tab) => loadedTabIds.includes(tab.id))
+          .map((tab) => {
+            const isActive = tab.id === activeTabId;
+            return (
+              <div
+                key={tab.id}
+                className={cn("absolute inset-0 overflow-hidden", !isActive && "hidden")}
+                aria-hidden={!isActive}
+              >
+                {renderTabBody(tab)}
+              </div>
+            );
+          })}
+      </PaneSizeContext.Provider>
     </div>
   );
 }
@@ -233,8 +259,16 @@ export default function ProHome() {
 
   if (!isDesktop) return null;
 
+  // Stable keys are essential: when the split collapses, the row's child
+  // list goes from [splitPane, divider, mainPane] (or the leading variant)
+  // to [mainPane]. Without keys, React would reuse the Pane instance at
+  // index 0 — repurposing the *split* pane's instance into the main pane,
+  // which strands the main pane's ResizeObserver/paneWidth on a now-
+  // unmounted DOM node and reparents the mail tab body (causing remount
+  // + stale "still-narrow" measurements after the split is closed).
   const mainPane = (
     <Pane
+      key="pane-main"
       paneId="main"
       tabs={mainTabs}
       activeTabId={activeMainTabId}
@@ -246,6 +280,7 @@ export default function ProHome() {
 
   const splitPane = isSplit ? (
     <Pane
+      key="pane-split"
       paneId="split"
       tabs={splitTabs}
       activeTabId={activeSplitTabId}
@@ -257,6 +292,7 @@ export default function ProHome() {
 
   const splitDivider = isSplit ? (
     <div
+      key="pane-divider"
       aria-hidden="true"
       className="flex-shrink-0 w-px bg-transparent"
       style={{ borderLeft: '1px solid rgba(128, 128, 128, 0.3)' }}
diff --git a/components/layout/mobile-header.tsx b/components/layout/mobile-header.tsx
index a3ce30f6..f4cd3f60 100644
--- a/components/layout/mobile-header.tsx
+++ b/components/layout/mobile-header.tsx
@@ -3,6 +3,7 @@
 import { Menu, ArrowLeft, Plus, Search, X } from "lucide-react";
 import { Button } from "@/components/ui/button";
 import { useUIStore } from "@/stores/ui-store";
+import { useIsDesktop } from "@/hooks/use-media-query";
 import { cn } from "@/lib/utils";
 import { useTranslations } from "next-intl";
 
@@ -25,6 +26,12 @@ export function MobileHeader({
 }: MobileHeaderProps) {
   const t = useTranslations('sidebar');
   const { toggleSidebar, goBack, sidebarOpen } = useUIStore();
+  // Pane-aware: in Pro split mode the viewport is desktop-wide while the
+  // pane is narrow. The Tailwind `lg:hidden` variant alone would never fire
+  // there, so we additionally hide via JS when the surrounding pane is
+  // desktop-sized. Outside of Pro this still returns the viewport value.
+  const isPaneDesktop = useIsDesktop();
+  if (isPaneDesktop) return null;
 
   const handleLeftAction = () => {
     if (showBack && onBack) {
@@ -40,7 +47,6 @@ export function MobileHeader({
     <header
       className={cn(
         "flex items-center justify-between px-4 h-14 border-b border-border bg-background shrink-0",
-        "lg:hidden", // Only visible on mobile/tablet
         className
       )}
     >
@@ -118,12 +124,13 @@ export function MobileViewerHeader({
   className,
 }: MobileViewerHeaderProps) {
   const t = useTranslations('sidebar');
+  const isPaneDesktop = useIsDesktop();
+  if (isPaneDesktop) return null;
 
   return (
     <header
       className={cn(
         "flex items-center justify-between px-2 h-14 border-b border-border bg-background shrink-0",
-        "lg:hidden", // Only visible on mobile/tablet
         className
       )}
     >
diff --git a/hooks/use-media-query.ts b/hooks/use-media-query.ts
index 0cf52373..bcec359a 100644
--- a/hooks/use-media-query.ts
+++ b/hooks/use-media-query.ts
@@ -2,6 +2,7 @@
 
 import { useCallback, useEffect, useSyncExternalStore } from "react";
 import { useUIStore } from "@/stores/ui-store";
+import { usePaneSize } from "@/hooks/use-pane-size";
 
 // Tailwind v4 breakpoints
 const BREAKPOINTS = {
@@ -38,12 +39,38 @@ export function useMediaQuery(query: string): boolean {
   return useSyncExternalStore(subscribe, getSnapshot, getMediaQueryServerSnapshot);
 }
 
+/**
+ * When the Pro shell renders a page inside a (possibly split) pane, that pane
+ * publishes its measured width via `PaneSizeContext`. Inner pages should
+ * branch their layout against the pane width — not the full viewport — so a
+ * narrow pane gets the mobile/tablet layout instead of overflowing.
+ *
+ * Returns `null` when no pane size is published, signalling the caller to
+ * fall back to `window.matchMedia`.
+ */
+function classifyPane(paneWidth: number | null) {
+  if (paneWidth === null) return null;
+  return {
+    isMobile: paneWidth < BREAKPOINTS.md,
+    isTablet: paneWidth >= BREAKPOINTS.md && paneWidth < BREAKPOINTS.lg,
+    isDesktop: paneWidth >= BREAKPOINTS.lg,
+  };
+}
+
 /**
  * Hook to detect device type and sync with UI store
  * Uses Tailwind breakpoints: mobile < 768px, tablet 768-1024px, desktop > 1024px
+ *
+ * When invoked inside a Pro pane, the returned values reflect the pane's
+ * width instead of the window's. The global UI store is NOT updated in that
+ * case — two split panes would otherwise fight to write conflicting values,
+ * and the store is meant to mirror the actual viewport for callers that read
+ * it directly (mobile navigation helpers etc.).
  */
 export function useDeviceDetection() {
   const { setDeviceType, isMobile, isTablet, isDesktop } = useUIStore();
+  const paneWidth = usePaneSize();
+  const paneClassification = classifyPane(paneWidth);
 
   const isMobileQuery = useMediaQuery(`(max-width: ${BREAKPOINTS.md - 1}px)`);
   const isTabletQuery = useMediaQuery(
@@ -52,9 +79,11 @@ export function useDeviceDetection() {
   const isDesktopQuery = useMediaQuery(`(min-width: ${BREAKPOINTS.lg}px)`);
 
   useEffect(() => {
+    if (paneClassification) return;
     setDeviceType(isMobileQuery, isTabletQuery, isDesktopQuery);
-  }, [isMobileQuery, isTabletQuery, isDesktopQuery, setDeviceType]);
+  }, [isMobileQuery, isTabletQuery, isDesktopQuery, setDeviceType, paneClassification]);
 
+  if (paneClassification) return paneClassification;
   return { isMobile, isTablet, isDesktop };
 }
 
@@ -62,19 +91,29 @@ export function useDeviceDetection() {
  * Convenience hooks for specific breakpoints
  */
 export function useIsMobile() {
-  return useMediaQuery(`(max-width: ${BREAKPOINTS.md - 1}px)`);
+  const paneWidth = usePaneSize();
+  const viewport = useMediaQuery(`(max-width: ${BREAKPOINTS.md - 1}px)`);
+  return paneWidth !== null ? paneWidth < BREAKPOINTS.md : viewport;
 }
 
 export function useIsTablet() {
-  return useMediaQuery(
+  const paneWidth = usePaneSize();
+  const viewport = useMediaQuery(
     `(min-width: ${BREAKPOINTS.md}px) and (max-width: ${BREAKPOINTS.lg - 1}px)`
   );
+  return paneWidth !== null
+    ? paneWidth >= BREAKPOINTS.md && paneWidth < BREAKPOINTS.lg
+    : viewport;
 }
 
 export function useIsDesktop() {
-  return useMediaQuery(`(min-width: ${BREAKPOINTS.lg}px)`);
+  const paneWidth = usePaneSize();
+  const viewport = useMediaQuery(`(min-width: ${BREAKPOINTS.lg}px)`);
+  return paneWidth !== null ? paneWidth >= BREAKPOINTS.lg : viewport;
 }
 
 export function useBreakpoint(breakpoint: keyof typeof BREAKPOINTS) {
-  return useMediaQuery(`(min-width: ${BREAKPOINTS[breakpoint]}px)`);
+  const paneWidth = usePaneSize();
+  const viewport = useMediaQuery(`(min-width: ${BREAKPOINTS[breakpoint]}px)`);
+  return paneWidth !== null ? paneWidth >= BREAKPOINTS[breakpoint] : viewport;
 }
diff --git a/hooks/use-pane-size.ts b/hooks/use-pane-size.ts
new file mode 100644
index 00000000..1b952073
--- /dev/null
+++ b/hooks/use-pane-size.ts
@@ -0,0 +1,14 @@
+"use client";
+
+import { createContext, useContext } from "react";
+
+/**
+ * Width of the pane that's hosting the current subtree, in CSS pixels.
+ * `null` means "no pane is providing a size" — fall back to viewport-based
+ * media queries. Set by the Pro shell on each split pane via ResizeObserver.
+ */
+export const PaneSizeContext = createContext<number | null>(null);
+
+export function usePaneSize(): number | null {
+  return useContext(PaneSizeContext);
+}