diff --git a/apps/readest-app/src/__tests__/utils/window.test.ts b/apps/readest-app/src/__tests__/utils/window.test.ts
index bf148582..65c0cce7 100644
--- a/apps/readest-app/src/__tests__/utils/window.test.ts
+++ b/apps/readest-app/src/__tests__/utils/window.test.ts
@@ -24,7 +24,7 @@ vi.mock('@/utils/event', () => ({
 
 import { getCurrentWindow } from '@tauri-apps/api/window';
 import { type as osType } from '@tauri-apps/plugin-os';
-import { tauriHandleOnCloseWindow } from '@/utils/window';
+import { tauriHandleOnCloseWindow, tauriHandleToggleFullScreen } from '@/utils/window';
 
 type CloseHandler = (event: { preventDefault: () => void }) => Promise<void> | void;
 
@@ -113,3 +113,62 @@ describe('tauriHandleOnCloseWindow', () => {
     expect(win.destroy).toHaveBeenCalled();
   });
 });
+
+function makeFullscreenWindow({
+  isFullscreen,
+  isMaximized,
+}: {
+  isFullscreen: boolean;
+  isMaximized: boolean;
+}) {
+  return {
+    isFullscreen: vi.fn().mockResolvedValue(isFullscreen),
+    isMaximized: vi.fn().mockResolvedValue(isMaximized),
+    setFullscreen: vi.fn().mockResolvedValue(undefined),
+    unmaximize: vi.fn().mockResolvedValue(undefined),
+    toggleMaximize: vi.fn().mockResolvedValue(undefined),
+    innerSize: vi.fn().mockResolvedValue({ width: 800, height: 600 }),
+    setSize: vi.fn().mockResolvedValue(undefined),
+  };
+}
+
+describe('tauriHandleToggleFullScreen', () => {
+  test('enters fullscreen when the window is maximized (Phosh / Windows-maximized case)', async () => {
+    // On Phosh the window is always maximized, and on Windows users often run
+    // maximized. The fullscreen button must still enter fullscreen instead of
+    // just unmaximizing the window (issue #4034).
+    vi.mocked(osType).mockReturnValue('linux');
+    const win = makeFullscreenWindow({ isFullscreen: false, isMaximized: true });
+    vi.mocked(getCurrentWindow).mockReturnValue(
+      win as unknown as ReturnType<typeof getCurrentWindow>,
+    );
+
+    await tauriHandleToggleFullScreen();
+
+    expect(win.setFullscreen).toHaveBeenCalledWith(true);
+  });
+
+  test('exits fullscreen when already fullscreen', async () => {
+    vi.mocked(osType).mockReturnValue('windows');
+    const win = makeFullscreenWindow({ isFullscreen: true, isMaximized: false });
+    vi.mocked(getCurrentWindow).mockReturnValue(
+      win as unknown as ReturnType<typeof getCurrentWindow>,
+    );
+
+    await tauriHandleToggleFullScreen();
+
+    expect(win.setFullscreen).toHaveBeenCalledWith(false);
+  });
+
+  test('enters fullscreen when neither maximized nor fullscreen', async () => {
+    vi.mocked(osType).mockReturnValue('macos');
+    const win = makeFullscreenWindow({ isFullscreen: false, isMaximized: false });
+    vi.mocked(getCurrentWindow).mockReturnValue(
+      win as unknown as ReturnType<typeof getCurrentWindow>,
+    );
+
+    await tauriHandleToggleFullScreen();
+
+    expect(win.setFullscreen).toHaveBeenCalledWith(true);
+  });
+});
diff --git a/apps/readest-app/src/utils/window.ts b/apps/readest-app/src/utils/window.ts
index 206bf7d0..aa78168e 100644
--- a/apps/readest-app/src/utils/window.ts
+++ b/apps/readest-app/src/utils/window.ts
@@ -71,12 +71,11 @@ export const tauriHandleOnCloseWindow = async (callback: () => void) => {
 export const tauriHandleToggleFullScreen = async () => {
   const currentWindow = getCurrentWindow();
   const isFullscreen = await currentWindow.isFullscreen();
-  const isMaximized = await currentWindow.isMaximized();
-  if (isMaximized) {
-    await currentWindow.unmaximize();
-  } else {
-    await currentWindow.setFullscreen(!isFullscreen);
-  }
+  // Toggle fullscreen regardless of the maximized state. Previously a maximized
+  // window was only unmaximized here, so the fullscreen button did nothing when
+  // the window was maximized, which is always the case on mobile shells like
+  // Phosh and common on Windows (issue #4034).
+  await currentWindow.setFullscreen(!isFullscreen);
   if ((await osType()) === 'linux') {
     linuxWindowRestoreTransparentBg();
   }