diff --git a/packages/app/e2e/regression/tabs.spec.ts b/packages/app/e2e/regression/tabs.spec.ts
index d62e8e13e21..01c20b7822f 100644
--- a/packages/app/e2e/regression/tabs.spec.ts
+++ b/packages/app/e2e/regression/tabs.spec.ts
@@ -435,6 +435,34 @@ for (const [screen, href] of [
   })
 }
 
+// macOS 26 traffic lights end 74px from the window edge. The collapsed toggle starts 16px past them, as the horizontal
+// titlebar's first control does, and a fullscreen window has no traffic lights to clear.
+for (const [window, fullscreen, start] of [
+  ["windowed", false, 90],
+  ["fullscreen", true, 18],
+] as const) {
+  test(`collapsed sidebar toggle clears the macOS traffic lights (${window})`, async ({ page }) => {
+    await mockWorkspace(page, {
+      name: "Tabs",
+      sessions: [a],
+      seed: { settings: { appearance: { tabLayout: "vertical", sidebarOpen: false } }, tabs: [a.id] },
+    })
+    await page.goto(
+      `/e2e/utils/macos-window.html?${new URLSearchParams({ server: SERVER, route: sessionHref(a.id), fullscreen: String(fullscreen) })}`,
+    )
+    await expectSessionTitle(page, a.title)
+
+    const toggle = await page.locator("main").getByRole("button", { name: "Toggle sidebar", exact: true }).boundingBox()
+    const project = await page.locator('[data-session-title] [data-slot="session-project-trigger"]').boundingBox()
+
+    if (!toggle || !project) throw new Error("collapsed sidebar header has no bounding boxes")
+    expect(toggle.x).toBe(start)
+    // The header's contents follow the toggle on its center line.
+    expect(project.x).toBe(start + 32)
+    expect(project.y + project.height / 2).toBe(toggle.y + toggle.height / 2)
+  })
+}
+
 test("vertical tab gaps hover and select the nearest row", async ({ page }) => {
   await mockWorkspace(page, {
     name: "Tabs",
diff --git a/packages/app/e2e/utils/macos-window.fixture.tsx b/packages/app/e2e/utils/macos-window.fixture.tsx
new file mode 100644
index 00000000000..e96f4c04aa4
--- /dev/null
+++ b/packages/app/e2e/utils/macos-window.fixture.tsx
@@ -0,0 +1,65 @@
+import { MemoryRouter, createMemoryHistory } from "@solidjs/router"
+import { render } from "solid-js/web"
+import type { Bridge } from "@opencode/gui-extensions/sdk/bridge"
+import { AppBaseProviders, AppInterface } from "../../src/app"
+import { PlatformProvider, type Platform } from "../../src/runtime/platform/platform"
+
+// The app in a macOS desktop window at `route`, so its layout leaves room for the native traffic lights. A fullscreen
+// window has none.
+export function mount(input: { server: string; route: string; fullscreen: boolean }) {
+  const root = document.getElementById("root")
+
+  if (!root) throw new Error("Missing fixture root")
+  const history = createMemoryHistory()
+  history.set({ value: input.route })
+  render(() => {
+    const unused = async () => {
+      throw new Error("Unexpected fixture action")
+    }
+
+    // No main-process extensions run in this fixture.
+    const bridge: Bridge = {
+      packaged: false,
+      call: unused,
+      subscribe: async () => ({ available: false }),
+      on: () => () => undefined,
+      embed: () => undefined,
+      capture: async () => undefined,
+      runMenubarItem: () => undefined,
+      configure: () => undefined,
+      manager: {
+        list: async () => [],
+        enable: async () => undefined,
+        disable: async () => undefined,
+        reload: async () => undefined,
+        install: async () => undefined,
+        remove: async () => undefined,
+        source: async () => "",
+        asset: () => "",
+      },
+    }
+
+    const platform: Platform = {
+      platform: "desktop",
+      os: "macos",
+      windowID: "macos-window-test",
+      windowFullscreen: () => input.fullscreen,
+      openExternal: () => undefined,
+      openDirectoryPickerDialog: async () => null,
+      notify: async () => undefined,
+      restart: unused,
+      extensions: bridge,
+    }
+
+    return (
+      <PlatformProvider value={platform}>
+        <AppBaseProviders locale="en">
+          <AppInterface
+            servers={[{ type: "http", displayName: "Local Server", http: { url: input.server } }]}
+            router={(props) => <MemoryRouter {...props} history={history} />}
+          />
+        </AppBaseProviders>
+      </PlatformProvider>
+    )
+  }, root)
+}
diff --git a/packages/app/e2e/utils/macos-window.html b/packages/app/e2e/utils/macos-window.html
new file mode 100644
index 00000000000..62b53e494f3
--- /dev/null
+++ b/packages/app/e2e/utils/macos-window.html
@@ -0,0 +1,18 @@
+<!doctype html>
+<html lang="en">
+  <head>
+    <meta charset="UTF-8" />
+  </head>
+  <body class="antialiased overscroll-none overflow-hidden">
+    <div id="root" class="flex flex-col h-dvh"></div>
+    <script type="module">
+      import { mount } from "./macos-window.fixture.tsx"
+      const query = new URLSearchParams(location.search)
+      mount({
+        server: query.get("server"),
+        route: query.get("route") ?? "/",
+        fullscreen: query.get("fullscreen") === "true",
+      })
+    </script>
+  </body>
+</html>
diff --git a/packages/app/src/shell/sidebar-toggle.tsx b/packages/app/src/shell/sidebar-toggle.tsx
index c0b564cd46e..61ccc8260ab 100644
--- a/packages/app/src/shell/sidebar-toggle.tsx
+++ b/packages/app/src/shell/sidebar-toggle.tsx
@@ -9,8 +9,13 @@ import { useLanguage } from "@/runtime/i18n/language"
 import { usePlatform } from "@/runtime/platform/platform"
 import { useSettings } from "@/settings/model"
 
-// Native controls: 14px left inset, two 20px button pitches, and a 14px button.
-export const macTrafficLightsBaseWidth = 68
+// The desktop window sets `trafficLightPosition: { x: 14, y: 14 }`. macOS 26 and later draw 14px buttons on a 23px
+// pitch, so the zoom button ends 74px from the window edge and the buttons center 21px from the top. macOS 15 and earlier
+// end at 68px and center at 22px.
+export const macTrafficLightsBaseWidth = 74
+
+// Matches the horizontal titlebar, whose first control starts 16px past the traffic lights.
+const macTrafficLightsGap = 16
 
 // The toggle's 28px plus a 4px gap before the header contents.
 const slotWidth = 32
@@ -65,9 +70,11 @@ function useCollapsedSlot() {
   return {
     visible: () => desktop() && !windows() && settings.appearance.tabLayout() !== "horizontal",
     open: settings.appearance.sidebarOpen,
-    // Keep the toggle clear of the traffic lights, which end 68px from the window edge.
+    // Lays the toggle out as if the traffic lights were the header's first control.
     width: () =>
-      mac() ? macTrafficLightsBaseWidth / (platform.webviewZoom?.() ?? 1) - contentStart + slotWidth : slotWidth,
+      mac()
+        ? macTrafficLightsBaseWidth / (platform.webviewZoom?.() ?? 1) + macTrafficLightsGap - contentStart + slotWidth
+        : slotWidth,
   }
 }
 
diff --git a/packages/app/vite.config.ts b/packages/app/vite.config.ts
index 87b1ce7a66b..de39bd9aea4 100644
--- a/packages/app/vite.config.ts
+++ b/packages/app/vite.config.ts
@@ -43,6 +43,7 @@ export default defineConfig({
               "e2e/utils/settings-wsl.html",
               "e2e/utils/app-direction.html",
               "e2e/utils/windows-menu.html",
+              "e2e/utils/macos-window.html",
             ],
           },
         }