diff --git a/packages/ui/src/shell/HeaderRow.tsx b/packages/ui/src/shell/HeaderRow.tsx
index cc43d53f4..66d88c34e 100644
--- a/packages/ui/src/shell/HeaderRow.tsx
+++ b/packages/ui/src/shell/HeaderRow.tsx
@@ -26,6 +26,7 @@ import { useWorkspace } from "@posthog/ui/features/workspace/useWorkspace";
 import { Tooltip } from "@posthog/ui/primitives/Tooltip";
 import { useAppView } from "@posthog/ui/router/useAppView";
 import { track } from "@posthog/ui/shell/analytics";
+import { getHeaderSidebarPanelLayout } from "@posthog/ui/shell/headerSidebarPanel";
 import { useHeaderStore } from "@posthog/ui/shell/headerStore";
 import { isMac, isWindows } from "@posthog/ui/utils/platform";
 import { Box, Flex } from "@radix-ui/themes";
@@ -162,9 +163,7 @@ function BluebirdButton() {
 }
 
 export const HEADER_HEIGHT = 36;
-const COLLAPSED_WIDTH = 110;
 const WINDOWS_TITLEBAR_INSET = 140;
-const MACOS_TRAFFIC_LIGHT_INSET = 70;
 
 export function HeaderRow() {
   const content = useHeaderStore((state) => state.content);
@@ -174,6 +173,11 @@ export function HeaderRow() {
   const sidebarWidth = useSidebarStore((state) => state.width);
   const isResizing = useSidebarStore((state) => state.isResizing);
   const setIsResizing = useSidebarStore((state) => state.setIsResizing);
+  const panel = getHeaderSidebarPanelLayout({
+    sidebarOpen,
+    sidebarWidth,
+    isMac,
+  });
 
   const activeTaskId = view.type === "task-detail" ? view.taskId : undefined;
   // Read the live task from the list cache instead of a stale snapshot off the
@@ -206,17 +210,20 @@ export function HeaderRow() {
     >
       <Flex
         align="center"
-        justify="end"
+        justify={panel.justify}
         px="2"
         pr="3"
         style={{
-          width: sidebarOpen
-            ? `${sidebarWidth}px`
-            : `${COLLAPSED_WIDTH + (isMac ? MACOS_TRAFFIC_LIGHT_INSET : 0)}px`,
-          minWidth: `${COLLAPSED_WIDTH + (isMac ? MACOS_TRAFFIC_LIGHT_INSET : 0)}px`,
+          width: panel.width,
+          minWidth: panel.minWidth,
+          paddingLeft: panel.paddingLeft,
           transition: isResizing ? "none" : "width 0.2s ease-in-out",
         }}
-        className="relative h-full gap-2 border-r border-r-(--gray-6)"
+        className={
+          panel.showBorder
+            ? "relative h-full gap-2 border-r border-r-(--gray-6)"
+            : "relative h-full gap-2"
+        }
       >
         <BluebirdButton />
         <SidebarTrigger />
diff --git a/packages/ui/src/shell/headerSidebarPanel.test.ts b/packages/ui/src/shell/headerSidebarPanel.test.ts
new file mode 100644
index 000000000..86ff9f456
--- /dev/null
+++ b/packages/ui/src/shell/headerSidebarPanel.test.ts
@@ -0,0 +1,60 @@
+import {
+  getHeaderSidebarPanelLayout,
+  type HeaderSidebarPanelLayout,
+} from "@posthog/ui/shell/headerSidebarPanel";
+import { describe, expect, it } from "vitest";
+
+describe("getHeaderSidebarPanelLayout", () => {
+  it.each<{
+    name: string;
+    input: { sidebarOpen: boolean; sidebarWidth: number; isMac: boolean };
+    expected: HeaderSidebarPanelLayout;
+  }>([
+    {
+      name: "open on macOS tracks the sidebar width and keeps the divider",
+      input: { sidebarOpen: true, sidebarWidth: 256, isMac: true },
+      expected: {
+        width: "256px",
+        minWidth: "180px",
+        paddingLeft: undefined,
+        justify: "end",
+        showBorder: true,
+      },
+    },
+    {
+      name: "open off macOS tracks the sidebar width",
+      input: { sidebarOpen: true, sidebarWidth: 300, isMac: false },
+      expected: {
+        width: "300px",
+        minWidth: "110px",
+        paddingLeft: undefined,
+        justify: "end",
+        showBorder: true,
+      },
+    },
+    {
+      name: "collapsed on macOS reserves the traffic-light strip as left padding and drops the divider",
+      input: { sidebarOpen: false, sidebarWidth: 256, isMac: true },
+      expected: {
+        width: "180px",
+        minWidth: "180px",
+        paddingLeft: "70px",
+        justify: "start",
+        showBorder: false,
+      },
+    },
+    {
+      name: "collapsed off macOS drops the divider and needs no traffic-light padding",
+      input: { sidebarOpen: false, sidebarWidth: 256, isMac: false },
+      expected: {
+        width: "110px",
+        minWidth: "110px",
+        paddingLeft: undefined,
+        justify: "start",
+        showBorder: false,
+      },
+    },
+  ])("$name", ({ input, expected }) => {
+    expect(getHeaderSidebarPanelLayout(input)).toEqual(expected);
+  });
+});
diff --git a/packages/ui/src/shell/headerSidebarPanel.ts b/packages/ui/src/shell/headerSidebarPanel.ts
new file mode 100644
index 000000000..15278099f
--- /dev/null
+++ b/packages/ui/src/shell/headerSidebarPanel.ts
@@ -0,0 +1,56 @@
+// The header's left region mirrors the sidebar: it holds the app-rail buttons
+// (Bluebird) plus the sidebar toggle and lines up with the sidebar body below
+// it. This helper derives its layout so the collapsed state stays clean — it
+// clears the macOS traffic lights and drops the divider that would otherwise
+// dangle as a stray edge over the page once the sidebar body has collapsed to
+// zero width.
+
+/** Width of the collapsed panel, sized to hold the app-rail + toggle buttons. */
+const COLLAPSED_WIDTH = 110;
+/**
+ * Width of the macOS traffic-light strip (close / minimize / zoom). Reserved as
+ * real left padding when collapsed so the buttons can never render underneath
+ * the window controls.
+ */
+const MACOS_TRAFFIC_LIGHT_INSET = 70;
+
+export interface HeaderSidebarPanelLayout {
+  /** Inline width / minWidth for the panel (animates between the two states). */
+  width: string;
+  minWidth: string;
+  /**
+   * Left padding reserving the macOS traffic-light strip, or `undefined` when no
+   * reservation is needed (open, or non-macOS).
+   */
+  paddingLeft: string | undefined;
+  /**
+   * Buttons hug the sidebar's right edge when open (matching the divider); when
+   * collapsed they hug the left, after the traffic-light inset, so a wide
+   * Bluebird label grows rightward into empty space rather than leftward under
+   * the window controls.
+   */
+  justify: "start" | "end";
+  /** The divider only belongs while the sidebar body is visible beneath it. */
+  showBorder: boolean;
+}
+
+export function getHeaderSidebarPanelLayout({
+  sidebarOpen,
+  sidebarWidth,
+  isMac,
+}: {
+  sidebarOpen: boolean;
+  sidebarWidth: number;
+  isMac: boolean;
+}): HeaderSidebarPanelLayout {
+  const collapsedWidth =
+    COLLAPSED_WIDTH + (isMac ? MACOS_TRAFFIC_LIGHT_INSET : 0);
+  return {
+    width: sidebarOpen ? `${sidebarWidth}px` : `${collapsedWidth}px`,
+    minWidth: `${collapsedWidth}px`,
+    paddingLeft:
+      !sidebarOpen && isMac ? `${MACOS_TRAFFIC_LIGHT_INSET}px` : undefined,
+    justify: sidebarOpen ? "end" : "start",
+    showBorder: sidebarOpen,
+  };
+}