diff --git a/packages/extension/src/services/chrome.ts b/packages/extension/src/services/chrome.ts
index e2d597d0..84b55401 100644
--- a/packages/extension/src/services/chrome.ts
+++ b/packages/extension/src/services/chrome.ts
@@ -446,7 +446,7 @@ export const openPopupWindow = async (
     current = { id: undefined, incognito: false } as chrome.windows.Window
   }
 
-  const screenSize = await getScreenSize()
+  const screenSize = await getScreenSize({ top, left })
 
   const type = param.type ?? POPUP_TYPE.POPUP
   const isFullscreen = param.windowState === WINDOW_STATE.FULLSCREEN
@@ -553,7 +553,7 @@ export const openPopupWindowMultiple = async (
   }
 
   const type = param.type ?? POPUP_TYPE.POPUP
-  const screenSize = await getScreenSize()
+  const screenSize = await getScreenSize({ top, left })
 
   const windows = await Promise.all(
     param.urls
diff --git a/packages/extension/src/services/screen.test.ts b/packages/extension/src/services/screen.test.ts
new file mode 100644
index 00000000..c0c671ac
--- /dev/null
+++ b/packages/extension/src/services/screen.test.ts
@@ -0,0 +1,160 @@
+import { describe, it, expect, vi, beforeEach } from "vitest"
+import { getScreenSize } from "./screen"
+
+// Mock isServiceWorker to control context
+vi.mock("@/lib/utils", () => ({
+  isServiceWorker: vi.fn(),
+}))
+
+import { isServiceWorker } from "@/lib/utils"
+
+// Minimal display info structure for testing
+const makeDisplay = (
+  left: number,
+  top: number,
+  width: number,
+  height: number,
+  isPrimary = false,
+): chrome.system.display.DisplayUnitInfo =>
+  ({
+    id: `display-${left}`,
+    name: `Display ${left}`,
+    isPrimary,
+    isEnabled: true,
+    bounds: { left, top, width, height },
+    workArea: { left, top, width, height },
+    overscan: { left: 0, top: 0, right: 0, bottom: 0 },
+    rotation: 0,
+    dpiX: 96,
+    dpiY: 96,
+    mirroringSourceId: "",
+    mirroringDestinationIds: [],
+    isUnified: false,
+    activeState: "active",
+    displayZoomFactor: 1,
+  }) as unknown as chrome.system.display.DisplayUnitInfo
+
+describe("getScreenSize", () => {
+  const primaryDisplay = makeDisplay(0, 0, 1920, 1080, true)
+  const secondaryDisplay = makeDisplay(1920, 0, 2560, 1440, false)
+
+  let mockGetInfo: ReturnType<typeof vi.fn>
+  let mockGetCurrent: ReturnType<typeof vi.fn>
+
+  beforeEach(() => {
+    vi.clearAllMocks()
+
+    mockGetInfo = vi.fn().mockResolvedValue([primaryDisplay, secondaryDisplay])
+    mockGetCurrent = vi
+      .fn()
+      .mockResolvedValue({ left: 100, top: 100 } as chrome.windows.Window)
+
+    vi.stubGlobal("chrome", {
+      system: {
+        display: {
+          getInfo: mockGetInfo,
+        },
+      },
+      windows: {
+        getCurrent: mockGetCurrent,
+      },
+    })
+  })
+
+  describe("In service worker context", () => {
+    beforeEach(() => {
+      vi.mocked(isServiceWorker).mockReturnValue(true)
+    })
+
+    it("GSS-01: ヒントなし - getCurrent() の結果からディスプレイを特定する", async () => {
+      // getCurrent returns position on primary display
+      mockGetCurrent.mockResolvedValue({
+        left: 100,
+        top: 100,
+      } as chrome.windows.Window)
+
+      const result = await getScreenSize()
+
+      expect(result.left).toBe(0)
+      expect(result.top).toBe(0)
+      expect(result.width).toBe(1920)
+      expect(result.height).toBe(1080)
+    })
+
+    it("GSS-02: ヒントあり（プライマリディスプレイ上の座標）- プライマリディスプレイを返す", async () => {
+      const result = await getScreenSize({ top: 200, left: 300 })
+
+      // Should identify primary display (0,0,1920,1080)
+      expect(result.left).toBe(0)
+      expect(result.top).toBe(0)
+      expect(result.width).toBe(1920)
+      expect(result.height).toBe(1080)
+      // getCurrent should NOT be called when hint is provided
+      expect(mockGetCurrent).not.toHaveBeenCalled()
+    })
+
+    it("GSS-03: ヒントあり（セカンダリディスプレイ上の座標）- セカンダリディスプレイを返す", async () => {
+      // Position on secondary display (left=1920, right=4480)
+      const result = await getScreenSize({ top: 100, left: 2000 })
+
+      // Should identify secondary display (1920,0,2560,1440)
+      expect(result.left).toBe(1920)
+      expect(result.top).toBe(0)
+      expect(result.width).toBe(2560)
+      expect(result.height).toBe(1440)
+      // getCurrent should NOT be called when hint is provided
+      expect(mockGetCurrent).not.toHaveBeenCalled()
+    })
+
+    it("GSS-04: マルチディスプレイ環境でヒントがプライマリにない場合もセカンダリを正しく返す", async () => {
+      // Simulate: getCurrent returns primary window, but popup hint is on secondary display
+      mockGetCurrent.mockResolvedValue({
+        left: 100, // on primary display
+        top: 100,
+      } as chrome.windows.Window)
+
+      const result = await getScreenSize({ top: 500, left: 3000 })
+
+      // Should correctly identify secondary display, not primary
+      expect(result.left).toBe(1920)
+      expect(result.top).toBe(0)
+      expect(result.width).toBe(2560)
+      expect(result.height).toBe(1440)
+    })
+
+    it("GSS-05: ヒントがいずれのディスプレイにも含まれない場合、プライマリを返す", async () => {
+      // Position doesn't match any display
+      const result = await getScreenSize({ top: -100, left: -200 })
+
+      // Should fall back to primary display
+      expect(result.left).toBe(0)
+      expect(result.top).toBe(0)
+      expect(result.width).toBe(1920)
+      expect(result.height).toBe(1080)
+    })
+  })
+
+  describe("In content script context (non-service-worker)", () => {
+    beforeEach(() => {
+      vi.mocked(isServiceWorker).mockReturnValue(false)
+
+      // Mock window.screen
+      vi.stubGlobal("window", {
+        screen: {
+          width: 1920,
+          height: 1080,
+          availLeft: 0,
+          availTop: 0,
+        },
+      })
+    })
+
+    it("GSS-06: window.screen の値を返す（ヒントは無視）", async () => {
+      const result = await getScreenSize({ top: 500, left: 3000 })
+
+      expect(result.width).toBe(1920)
+      expect(result.height).toBe(1080)
+    })
+  })
+})
+
diff --git a/packages/extension/src/services/screen.ts b/packages/extension/src/services/screen.ts
index 63f1bbcd..cc864239 100644
--- a/packages/extension/src/services/screen.ts
+++ b/packages/extension/src/services/screen.ts
@@ -37,29 +37,36 @@ export async function getWindowPosition(): Promise<WindowPosition> {
   }
 }
 
-export async function getScreenSize(): Promise<ScreenSize> {
+export async function getScreenSize(hint?: {
+  top: number
+  left: number
+}): Promise<ScreenSize> {
   if (isServiceWorker()) {
     try {
       // For background_script.ts
-      const [displays, currentWindow] = await Promise.all([
-        chrome.system.display.getInfo(),
-        chrome.windows.getCurrent(),
-      ])
+      const displays = await chrome.system.display.getInfo()
 
-      let targetDisplay
-      const currentWindowLeft = currentWindow.left
-      const currentWindowTop = currentWindow.top
+      let hintTop = hint?.top
+      let hintLeft = hint?.left
 
-      if (currentWindowLeft != null && currentWindowTop != null) {
-        // Find the monitor that contains the active window's left position
+      if (hintTop == null || hintLeft == null) {
+        // Fall back to current window position if no hint provided
+        const currentWindow = await chrome.windows.getCurrent()
+        hintLeft = currentWindow.left ?? undefined
+        hintTop = currentWindow.top ?? undefined
+      }
+
+      let targetDisplay
+      if (hintLeft != null && hintTop != null) {
+        // Find the monitor that contains the given position
         targetDisplay =
           displays.find((d) => {
             const a = d.workArea
             return (
-              currentWindowLeft >= a.left &&
-              currentWindowLeft < a.left + a.width &&
-              currentWindowTop >= a.top &&
-              currentWindowTop < a.top + a.height
+              hintLeft >= a.left &&
+              hintLeft < a.left + a.width &&
+              hintTop >= a.top &&
+              hintTop < a.top + a.height
             )
           }) ??
           displays.find((d) => d.isPrimary) ??