diff --git a/.github/workflows/linux-lua.yml b/.github/workflows/linux-lua.yml
index 723b568..6c7ac89 100644
--- a/.github/workflows/linux-lua.yml
+++ b/.github/workflows/linux-lua.yml
@@ -64,7 +64,7 @@ jobs:
       - name: Test native titlebar alignment
         if: runner.os == 'macOS'
         run: >-
-          cargo test --locked -p nebula --bin pebrel --features gpui-test-support gpui_shell::workspace::window_titlebar::macos::tests::
+          cargo test --locked -p nebula --bin pebrel --features gpui-test-support platform::window_chrome::macos::tests::
       - name: Test all Python helpers
         if: ${{ !cancelled() }}
         run: python -m unittest discover -s scripts/tests -v
diff --git a/nebula_app/src/gpui_shell/workspace/window_titlebar.rs b/nebula_app/src/gpui_shell/workspace/window_titlebar.rs
index 56c9377..aa6c875 100644
--- a/nebula_app/src/gpui_shell/workspace/window_titlebar.rs
+++ b/nebula_app/src/gpui_shell/workspace/window_titlebar.rs
@@ -1,8 +1,5 @@
 use super::*;
 
-#[cfg(target_os = "macos")]
-pub(super) mod macos;
-
 /// Prepaint records the actual pane column before any titlebar paint runs. Keep
 /// this cell for the workspace lifetime, so resizing and panel animations do not
 /// allocate a new shared slot or duplicate the body's layout calculations.
@@ -90,6 +87,7 @@ impl NebulaWorkspace {
         cx: &mut Context<Self>,
     ) -> gpui::Div {
         let top_tabs = self.tabs_position == nebula_settings::TabsPositionName::Top;
+        let native_layout = crate::platform::window_chrome::layout(window);
         let bar = TitleBar::new()
             // Leave 8px above and below the existing 32px controls.
             .h(px(48.0))
@@ -97,7 +95,7 @@ impl NebulaWorkspace {
             .when(settings_active, |bar| {
                 bar.border_b_1().border_color(crate::gpui_shell::theme::settings_hairline(cx))
             })
-            .when(top_tabs && !cfg!(target_os = "macos"), |bar| {
+            .when(top_tabs && native_layout.is_none(), |bar| {
                 bar.pl(px(top_tabs::TOP_TAB_LEFT_INSET))
             })
             .when(top_tabs, |bar| {
@@ -120,10 +118,10 @@ impl NebulaWorkspace {
 
         // AppKit owns both the control group and its geometry. Read the live
         // frames so system layout, resize and full-screen transitions agree.
-        #[cfg(target_os = "macos")]
-        let bar = {
-            let (height, inset) = macos::layout(window);
+        let bar = if let Some((height, inset)) = native_layout {
             bar.h(px(height)).pl(px(inset))
+        } else {
+            bar
         };
 
         div()
diff --git a/nebula_app/src/gpui_shell/workspace/windowing.rs b/nebula_app/src/gpui_shell/workspace/windowing.rs
index 657159d..29265b0 100644
--- a/nebula_app/src/gpui_shell/workspace/windowing.rs
+++ b/nebula_app/src/gpui_shell/workspace/windowing.rs
@@ -310,25 +310,15 @@ fn workspace_window_options(cx: &mut App, focus: bool, role: WindowRole) -> Wind
                     )
                 },
             );
-            WindowOptions {
+            crate::platform::window_chrome::configure_options(WindowOptions {
                 window_bounds: Some(WindowBounds::Windowed(bounds)),
                 window_min_size: Some(size(px(760.0), px(540.0)).min(&bounds.size)),
-                titlebar: Some(gpui::TitlebarOptions {
-                    // A manual traffic-light position makes GPUI rewrite AppKit's
-                    // titlebar container, breaking the system toolbar layout.
-                    traffic_light_position: if cfg!(target_os = "macos") {
-                        None
-                    } else {
-                        TitleBar::title_bar_options().traffic_light_position
-                    },
-                    ..TitleBar::title_bar_options()
-                }),
-                app_owns_titlebar_drag: cfg!(target_os = "macos"),
+                titlebar: Some(TitleBar::title_bar_options()),
                 app_id: Some("pebrel".to_owned()),
                 window_background: crate::gpui_shell::wallpaper::initial_background_appearance(),
                 focus,
                 ..Default::default()
-            }
+            })
         },
         #[cfg(windows)]
         WindowRole::QuickTerminal => {
@@ -392,8 +382,7 @@ fn open_workspace_window(
     let hwnd_out = hwnd_slot.clone();
     let handle = cx.open_window(options, move |window, cx| {
         window.set_window_title(crate::brand::NAME);
-        #[cfg(target_os = "macos")]
-        super::window_titlebar::macos::configure(window);
+        crate::platform::window_chrome::configure(window);
         *hwnd_out.borrow_mut() = native_hwnd(window).unwrap_or_default();
         #[cfg(windows)]
         crate::gpui_shell::set_native_window_icon(window);
diff --git a/nebula_app/src/platform/mod.rs b/nebula_app/src/platform/mod.rs
index a49c7db..868374f 100644
--- a/nebula_app/src/platform/mod.rs
+++ b/nebula_app/src/platform/mod.rs
@@ -27,6 +27,8 @@ pub mod notifications;
 pub mod shell;
 pub mod shell_integration;
 pub mod startup;
+#[cfg(feature = "gpui-shell")]
+pub(crate) mod window_chrome;
 
 pub use capabilities::CAPABILITIES;
 
diff --git a/nebula_app/src/platform/window_chrome.rs b/nebula_app/src/platform/window_chrome.rs
new file mode 100644
index 0000000..47424a0
--- /dev/null
+++ b/nebula_app/src/platform/window_chrome.rs
@@ -0,0 +1,43 @@
+//! Native window decoration belongs to the platform boundary. Workspace code
+//! supplies ordinary GPUI options and uses optional geometry without depending
+//! on AppKit types or duplicating platform detection.
+
+use gpui::{Window, WindowOptions};
+
+#[cfg(target_os = "macos")]
+mod macos;
+
+pub(crate) fn configure_options(options: WindowOptions) -> WindowOptions {
+    #[cfg(target_os = "macos")]
+    {
+        let mut options = options;
+        if let Some(titlebar) = options.titlebar.as_mut() {
+            // A manual position makes GPUI rewrite AppKit's titlebar container,
+            // breaking the native toolbar layout.
+            titlebar.traffic_light_position = None;
+        }
+        options.app_owns_titlebar_drag = true;
+        options
+    }
+    #[cfg(not(target_os = "macos"))]
+    {
+        options
+    }
+}
+
+pub(crate) fn configure(_window: &Window) {
+    #[cfg(target_os = "macos")]
+    macos::configure(_window);
+}
+
+/// Height and left inset when native controls determine the titlebar geometry.
+pub(crate) fn layout(_window: &Window) -> Option<(f32, f32)> {
+    #[cfg(target_os = "macos")]
+    {
+        Some(macos::layout(_window))
+    }
+    #[cfg(not(target_os = "macos"))]
+    {
+        None
+    }
+}
diff --git a/nebula_app/src/gpui_shell/workspace/window_titlebar/macos.rs b/nebula_app/src/platform/window_chrome/macos.rs
similarity index 98%
rename from nebula_app/src/gpui_shell/workspace/window_titlebar/macos.rs
rename to nebula_app/src/platform/window_chrome/macos.rs
index 8302715..016c80d 100644
--- a/nebula_app/src/gpui_shell/workspace/window_titlebar/macos.rs
+++ b/nebula_app/src/platform/window_chrome/macos.rs
@@ -14,7 +14,7 @@ fn native_window(window: &Window) -> Option<Retained<NSWindow>> {
     view.window()
 }
 
-pub(in super::super) fn configure(window: &Window) {
+pub(super) fn configure(window: &Window) {
     let Some(native) = native_window(window) else { return };
     let toolbar = NSToolbar::initWithIdentifier(
         NSToolbar::alloc(native.mtm()),
diff --git a/scripts/conformance/tests/test_posix_lifecycle.py b/scripts/conformance/tests/test_posix_lifecycle.py
index 8cd3cb8..817ed50 100644
--- a/scripts/conformance/tests/test_posix_lifecycle.py
+++ b/scripts/conformance/tests/test_posix_lifecycle.py
@@ -29,7 +29,11 @@ role = sys.argv[2]
 if role != 'grandchild':
     next_role = 'child' if role == 'parent' else 'grandchild'
     subprocess.Popen([sys.executable, '-I', '-S', __file__, str(root), next_role])
-(root / (role + '.pid')).write_text(str(os.getpid()))
+# Publish the PID only after the contents are complete. The parent polls for
+# file existence, so creating the final path first exposes an empty file.
+pid_path = root / (role + '.pid.tmp')
+pid_path.write_text(str(os.getpid()))
+pid_path.replace(root / (role + '.pid'))
 while True:
     if role == 'parent' and (root / 'exit').exists():
         code = root / 'exit-code'