fix: consistent mobile surface dimming and overlay scrim

Use a dark scrim for MobileOverlayPanel instead of a translucent background so
it dims the rest of the screen like the sessions sheet.

Rework MobileSurfaceShell to mirror that structure: the scrim is the fixed
parent's background with a quick fade (keeping it on a compositing layer iOS
Safari clips to the viewport), and the sheet is a flex child sized to leave the
top safe area dimmed — dropping the old top shadow that pooled at the corners.

Mount Files/Changes/Settings only while open so each recomputes its
safe-area layout fresh, fixing intermittent top-inset dimming, and drop the
dim behind the header overflow menu to avoid a scrim-to-scrim flicker on
select.

packages/ui/src/apps/MobileApp.tsx

@@ -99,7 +99,7 @@ const MobileOverflowMenu: React.FC<{
     <div className="fixed inset-0 z-50" role="dialog" aria-modal="true" aria-label={t('mobile.menu.titleAria')}>
       <button
         type="button"
-        className="absolute inset-0 cursor-default bg-[rgb(0_0_0_/_0.25)]"
+        className="absolute inset-0 cursor-default"
         aria-label={t('mobile.surface.closeAria')}
         onClick={onClose}
       />
@@ -286,47 +286,57 @@ const MobileShell: React.FC = () => {
           <MobileSessionsSheet open={sessionsSheetOpen} onOpenChange={setSessionsSheetOpen} />
         ) : null}
 
-        <MobileSurfaceShell
-          open={filesOpen}
-          onClose={() => setFilesOpen(false)}
-          ariaLabel={t('mobile.menu.files')}
-          headerless
-        >
-          <ErrorBoundary>
-            <MobileFilesSurface onClose={() => setFilesOpen(false)} />
-          </ErrorBoundary>
-        </MobileSurfaceShell>
+        {/* Mounted only while open (like the sessions sheet) so each surface
+            computes its safe-area / fixed-position layout fresh on open. Keeping
+            them always-mounted left a stale startup layout, which made the
+            top-inset dimming appear only intermittently on iOS. */}
+        {filesOpen ? (
+          <MobileSurfaceShell
+            open
+            onClose={() => setFilesOpen(false)}
+            ariaLabel={t('mobile.menu.files')}
+            headerless
+          >
+            <ErrorBoundary>
+              <MobileFilesSurface onClose={() => setFilesOpen(false)} />
+            </ErrorBoundary>
+          </MobileSurfaceShell>
+        ) : null}
 
-        <MobileSurfaceShell
-          open={changesOpen}
-          onClose={closeChanges}
-          ariaLabel={t('mobile.menu.changes')}
-          headerless
-        >
-          <ErrorBoundary>
-            <MobileChangesSurface
-              onClose={closeChanges}
-              initialDiffPath={pendingChangesDiff?.path ?? null}
-              initialDiffStaged={pendingChangesDiff?.staged === true}
-            />
-          </ErrorBoundary>
-        </MobileSurfaceShell>
+        {changesOpen ? (
+          <MobileSurfaceShell
+            open
+            onClose={closeChanges}
+            ariaLabel={t('mobile.menu.changes')}
+            headerless
+          >
+            <ErrorBoundary>
+              <MobileChangesSurface
+                onClose={closeChanges}
+                initialDiffPath={pendingChangesDiff?.path ?? null}
+                initialDiffStaged={pendingChangesDiff?.staged === true}
+              />
+            </ErrorBoundary>
+          </MobileSurfaceShell>
+        ) : null}
 
-        <MobileSurfaceShell
-          open={settingsOpen}
-          onClose={() => setSettingsOpen(false)}
-          ariaLabel={t('mobile.menu.settings')}
-          headerless
-        >
-          <ErrorBoundary>
-            <SettingsView
-              forceMobile
-              isWindowed
-              visiblePageSlugs={[...MOBILE_SETTINGS_PAGES]}
-              onClose={() => setSettingsOpen(false)}
-            />
-          </ErrorBoundary>
-        </MobileSurfaceShell>
+        {settingsOpen ? (
+          <MobileSurfaceShell
+            open
+            onClose={() => setSettingsOpen(false)}
+            ariaLabel={t('mobile.menu.settings')}
+            headerless
+          >
+            <ErrorBoundary>
+              <SettingsView
+                forceMobile
+                isWindowed
+                visiblePageSlugs={[...MOBILE_SETTINGS_PAGES]}
+                onClose={() => setSettingsOpen(false)}
+              />
+            </ErrorBoundary>
+          </MobileSurfaceShell>
+        ) : null}
       </div>
     </DedicatedMobileAppProvider>
   );

packages/ui/src/apps/MobileSurfaceShell.tsx

@@ -69,7 +69,7 @@ export const MobileSurfaceShell: React.FC<MobileSurfaceShellProps> = ({
       return () => window.clearTimeout(id);
     }
     setEntered(false);
-    const id = window.setTimeout(() => setMounted(false), 220);
+    const id = window.setTimeout(() => setMounted(false), 300);
     return () => window.clearTimeout(id);
   }, [open]);
 
@@ -180,31 +180,32 @@ export const MobileSurfaceShell: React.FC<MobileSurfaceShellProps> = ({
   return createPortal(
     <div
       className={cn(
-        'fixed inset-0 z-50 flex items-end',
-        'bg-[rgb(0_0_0_/_0.45)]',
+        'fixed inset-0 z-50 flex flex-col bg-[rgb(0_0_0_/_0.45)]',
+        // The opacity transition keeps the scrim on its own compositing layer,
+        // which iOS Safari clips to the viewport — without it, a static scrim
+        // bleeds the dim into the bottom toolbar overscroll zone. Quick fade so
+        // it still feels near-instant.
         'transition-opacity duration-200 ease-out',
         entered ? 'opacity-100' : 'opacity-0',
       )}
       role="dialog"
       aria-modal="true"
       aria-label={ariaLabel}
+      onClick={onClose}
     >
-      <button
-        type="button"
-        className="absolute inset-0 cursor-default"
-        aria-label={t('mobile.surface.closeAria')}
-        onClick={onClose}
-      />
+      {/* Sheet is a normal flex child — mirroring MobileOverlayPanel. */}
       <section
         ref={surfaceRef}
-        className="relative flex h-[100dvh] w-full flex-col overflow-hidden rounded-t-[20px] border-t border-border/40 bg-background text-foreground shadow-[0_-12px_48px_rgb(0_0_0_/_0.35)] will-change-transform"
+        className="mt-auto flex min-h-0 w-full flex-col overflow-hidden rounded-t-[20px] border-t border-border/40 bg-background text-foreground"
         tabIndex={-1}
+        onClick={(event) => event.stopPropagation()}
         style={{
+          // Sized to leave the top safe area uncovered so the scrim dims it.
+          height: 'calc(100% - var(--oc-safe-area-top, 0px))',
           transform: visualTransform,
           transition: isDraggingRef.current
             ? 'none'
-            : 'transform 220ms cubic-bezier(0.32, 0.72, 0, 1)',
-          paddingTop: 'var(--oc-safe-area-top, 0px)',
+            : 'transform 300ms cubic-bezier(0.32, 0.72, 0, 1)',
         }}
       >
         <div

packages/ui/src/components/ui/MobileOverlayPanel.tsx

@@ -70,7 +70,7 @@ export const MobileOverlayPanel: React.FC<MobileOverlayPanelProps> = ({
 
   const content = (
     <div
-      className="fixed inset-0 z-50 flex flex-col bg-background/70"
+      className="fixed inset-0 z-50 flex flex-col bg-[rgb(0_0_0_/_0.45)]"
       role="dialog"
       aria-modal="true"
       onClick={onClose}