fix(window): harden frame geometry across window states

- Remove rounded renderer corners from opaque Linux window surfaces.
- Track maximized state with race-safe native reads for main and Properties windows.
- Flatten maximized Windows frames without changing macOS AppKit zoom contours.
- Make early platform detection explicit, testable, and fail closed on unsupported targets.

src/App.tsx

@@ -57,6 +57,7 @@ import {
 } from './utils/schedulerControl';
 import { createSerialTaskQueue } from './utils/serialTaskQueue';
 import { useWindowFocusState } from './utils/windowFocus';
+import { useWindowMaximizedState } from './utils/windowMaximized';
 
 const loadSettingsView = () => import('./components/SettingsView');
 const loadSchedulerView = () => import('./components/SchedulerView');
@@ -189,6 +190,7 @@ function App() {
   const { i18n, t } = useTranslation();
   const platform = usePlatformInfo();
   const isWindowActive = useWindowFocusState();
+  const isWindowMaximized = useWindowMaximizedState();
   const [filter, setFilter] = useState<SidebarFilter>('all');
   const [downloadTableSummary, setDownloadTableSummary] = useState<DownloadTableStatusSummary | null>(null);
   const [coreReady, setCoreReady] = useState(false);
@@ -1222,7 +1224,7 @@ function App() {
   }, [autoAddClipboardLinks, coreReady, showKeychainModal]);
 
   return (
-    <div data-window-active={isWindowActive ? 'true' : 'false'} className={`app-shell app-shell--style-${windowControlStyle} flex h-screen w-screen overflow-hidden text-text-primary ${
+    <div data-window-active={isWindowActive ? 'true' : 'false'} data-window-maximized={isWindowMaximized ? 'true' : 'false'} className={`app-shell flex h-screen w-screen overflow-hidden text-text-primary ${
       isSidebarOnRight ? 'app-shell--sidebar-right' : 'app-shell--sidebar-left'
     } ${
       hasWindowChrome ? 'app-shell--window-chrome' : ''

src/components/PropertiesWindowApp.tsx

@@ -57,6 +57,7 @@ import { isTorrentLiveStatus } from '../utils/propertiesTorrentLifecycle';
 import { isTorrentWaitingForPeers } from '../utils/torrentPresentation';
 import { copyTorrentFilePath } from '../utils/torrentFilePath';
 import { useWindowFocusState } from '../utils/windowFocus';
+import { useWindowMaximizedState } from '../utils/windowMaximized';
 import { WindowControls } from './WindowControls';
 import {
   TORRENT_ENCRYPTION_POLICY_DISABLED,
@@ -204,6 +205,7 @@ const propertiesDiagnosticLifecycleKey = (snapshot: PropertiesSnapshot): string
 export const PropertiesWindowApp = () => {
   const { t } = useTranslation();
   const isWindowActive = useWindowFocusState();
+  const isWindowMaximized = useWindowMaximizedState();
   const translationRef = useRef(t);
   translationRef.current = t;
   const currentWindow = useMemo(() => getCurrentWindow(), []);
@@ -1138,6 +1140,7 @@ export const PropertiesWindowApp = () => {
         className={windowShellClassName}
         style={windowShellStyle}
         data-window-active={isWindowActive ? 'true' : 'false'}
+        data-window-maximized={isWindowMaximized ? 'true' : 'false'}
         aria-labelledby="properties-window-title"
       >
         <WindowControls side={windowChrome.side} controlStyle={windowChrome.controlStyle} />
@@ -1253,6 +1256,7 @@ export const PropertiesWindowApp = () => {
       className={windowShellClassName}
       style={windowShellStyle}
       data-window-active={isWindowActive ? 'true' : 'false'}
+      data-window-maximized={isWindowMaximized ? 'true' : 'false'}
       aria-labelledby="properties-window-title"
     >
       <WindowControls side={windowChrome.side} controlStyle={windowChrome.controlStyle} />

src/index.css

@@ -2373,6 +2373,19 @@ html[data-list-density="relaxed"] {
     border-width: 0.5px;
   }
 
+  /* Linux uses an opaque GTK/WebKit surface, so renderer-only curves would
+     expose square native backing pixels. Maximized Windows surfaces likewise
+     need to meet the work area instead of leaving transparent corner cutouts.
+     macOS zoomed windows retain their native rounded AppKit contour. */
+  html[data-platform="linux"] :is(.app-shell, .properties-window-shell),
+  html[data-platform="windows"] :is(.app-shell, .properties-window-shell)[data-window-maximized="true"] {
+    border-radius: 0;
+  }
+
+  html[data-platform="windows"] :is(.app-shell, .properties-window-shell)[data-window-maximized="true"] {
+    border-color: transparent;
+  }
+
   @media (forced-colors: active) {
     .app-shell,
     .properties-window-shell {

src/main.tsx

@@ -14,7 +14,10 @@ import { error as logError, warn as logWarn, initLogger } from "./utils/logger";
 import { getCurrentWindow } from '@tauri-apps/api/window';
 import { invokeCommand as invoke } from './ipc';
 import { useWindowFocusState } from './utils/windowFocus';
-import './utils/platform';
+import { syncPlatformDatasetFromUserAgent } from './utils/platform';
+import { useWindowMaximizedState } from './utils/windowMaximized';
+
+syncPlatformDatasetFromUserAgent(navigator.userAgent);
 
 const isPropertiesWindow = getCurrentWindow().label.startsWith('properties-');
 
@@ -86,8 +89,9 @@ const renderRoot = (RootComponent: ComponentType) => {
 
 const PropertiesStartupFailure = () => {
   const isWindowActive = useWindowFocusState();
+  const isWindowMaximized = useWindowMaximizedState();
   return (
-    <main data-window-active={isWindowActive ? 'true' : 'false'} className="properties-window-shell flex h-screen min-h-0 flex-col items-center justify-center gap-4 bg-main-bg p-6 text-text-primary">
+    <main data-window-active={isWindowActive ? 'true' : 'false'} data-window-maximized={isWindowMaximized ? 'true' : 'false'} className="properties-window-shell flex h-screen min-h-0 flex-col items-center justify-center gap-4 bg-main-bg p-6 text-text-primary">
       <p role="alert">Download Properties could not be loaded.</p>
       <button
         type="button"
@@ -106,8 +110,9 @@ const PropertiesStartupFailure = () => {
 
 const MainStartupFailure = () => {
   const isWindowActive = useWindowFocusState();
+  const isWindowMaximized = useWindowMaximizedState();
   return (
-    <main data-window-active={isWindowActive ? 'true' : 'false'} className="app-shell flex h-screen min-h-0 flex-col items-center justify-center gap-4 bg-main-bg p-6 text-text-primary">
+    <main data-window-active={isWindowActive ? 'true' : 'false'} data-window-maximized={isWindowMaximized ? 'true' : 'false'} className="app-shell flex h-screen min-h-0 flex-col items-center justify-center gap-4 bg-main-bg p-6 text-text-primary">
       <p role="alert">Firelink could not be loaded.</p>
       <button
         type="button"

src/utils/platform.test.ts

@@ -1,5 +1,10 @@
 import { describe, expect, it } from 'vitest';
-import { shouldUseCustomWindowControls, syncPlatformDataset } from './platform';
+import {
+  inferDesktopPlatform,
+  shouldUseCustomWindowControls,
+  syncPlatformDataset,
+  syncPlatformDatasetFromUserAgent,
+} from './platform';
 
 describe('shouldUseCustomWindowControls', () => {
   it('keeps custom controls present while Windows/Linux detection is unresolved', () => {
@@ -35,17 +40,39 @@ describe('syncPlatformDataset', () => {
     expect(mockDocument.documentElement.dataset.platform).toBe('linux');
   });
 
-  it('ignores unknown or unsupported platforms', () => {
+  it('clears a stale platform when authoritative detection is unknown or unsupported', () => {
     const mockDocument = { documentElement: { dataset: { platform: 'macos' } } };
 
     syncPlatformDataset('unknown', mockDocument);
-    expect(mockDocument.documentElement.dataset.platform).toBe('macos');
+    expect(mockDocument.documentElement.dataset.platform).toBeUndefined();
 
+    mockDocument.documentElement.dataset.platform = 'macos';
     syncPlatformDataset('android', mockDocument);
-    expect(mockDocument.documentElement.dataset.platform).toBe('macos');
+    expect(mockDocument.documentElement.dataset.platform).toBeUndefined();
   });
 
   it('safely handles missing document in headless environments', () => {
     expect(() => syncPlatformDataset('macos', undefined)).not.toThrow();
   });
 });
+
+describe('inferDesktopPlatform', () => {
+  it('identifies supported desktop webviews', () => {
+    expect(inferDesktopPlatform('Mozilla/5.0 (Macintosh; Intel Mac OS X 14_0)')).toBe('macos');
+    expect(inferDesktopPlatform('Mozilla/5.0 (Windows NT 10.0; Win64; x64)')).toBe('windows');
+    expect(inferDesktopPlatform('Mozilla/5.0 (X11; Linux x86_64)')).toBe('linux');
+  });
+
+  it('does not misclassify mobile user agents that contain desktop tokens', () => {
+    expect(inferDesktopPlatform('Mozilla/5.0 (Linux; Android 14; Mobile)')).toBe('unknown');
+    expect(inferDesktopPlatform('Mozilla/5.0 (Macintosh; iPad; Mobile)')).toBe('unknown');
+  });
+
+  it('synchronizes the inferred platform before the renderer mounts', () => {
+    const mockDocument = { documentElement: { dataset: {} as Record<string, string | undefined> } };
+
+    syncPlatformDatasetFromUserAgent('Mozilla/5.0 (Windows NT 10.0; Win64; x64)', mockDocument);
+
+    expect(mockDocument.documentElement.dataset.platform).toBe('windows');
+  });
+});

src/utils/platform.ts

@@ -35,18 +35,23 @@ export const syncPlatformDataset = (
   if (!targetDocument) return;
   if (os === 'macos' || os === 'windows' || os === 'linux') {
     targetDocument.documentElement.dataset.platform = os;
+  } else {
+    delete targetDocument.documentElement.dataset.platform;
   }
 };
 
-if (typeof document !== 'undefined' && typeof navigator !== 'undefined') {
-  if (/Macintosh|Mac OS X/i.test(navigator.userAgent)) {
-    syncPlatformDataset('macos');
-  } else if (/Windows/i.test(navigator.userAgent)) {
-    syncPlatformDataset('windows');
-  } else if (/Linux/i.test(navigator.userAgent)) {
-    syncPlatformDataset('linux');
-  }
-}
+export const inferDesktopPlatform = (userAgent: string): PlatformInfo['os'] => {
+  if (/Android|iPhone|iPad|iPod|Mobile/i.test(userAgent)) return 'unknown';
+  if (/Macintosh|Mac OS X/i.test(userAgent)) return 'macos';
+  if (/Windows/i.test(userAgent)) return 'windows';
+  if (/Linux/i.test(userAgent)) return 'linux';
+  return 'unknown';
+};
+
+export const syncPlatformDatasetFromUserAgent = (
+  userAgent: string,
+  targetDocument: TargetDocument | undefined = typeof document !== 'undefined' ? document : undefined,
+): void => syncPlatformDataset(inferDesktopPlatform(userAgent), targetDocument);
 
 export const getPlatformInfo = (): Promise<PlatformInfo> => {
   if (cached) return Promise.resolve(cached);

src/utils/windowMaximized.test.ts

@@ -0,0 +1,88 @@
+import { describe, expect, it, vi } from 'vitest';
+import { subscribeToWindowMaximized, type WindowMaximizedSource } from './windowMaximized';
+
+type WindowStateDisposer = () => void | Promise<void>;
+
+const deferred = <T,>() => {
+  let resolve!: (value: T) => void;
+  const promise = new Promise<T>(settle => { resolve = settle; });
+  return { promise, resolve };
+};
+
+describe('window maximized state', () => {
+  it('reads state immediately and again after listener registration closes the startup gap', async () => {
+    const onChange = vi.fn();
+    const source: WindowMaximizedSource = {
+      isMaximized: vi.fn()
+        .mockResolvedValueOnce(false)
+        .mockResolvedValueOnce(true),
+      listenResized: vi.fn(async () => vi.fn()),
+    };
+
+    const dispose = subscribeToWindowMaximized(source, onChange);
+
+    await vi.waitFor(() => expect(onChange).toHaveBeenLastCalledWith(true));
+    expect(source.isMaximized).toHaveBeenCalledTimes(2);
+    dispose();
+  });
+
+  it('keeps the newest resize state when native reads resolve out of order', async () => {
+    const initial = deferred<boolean>();
+    const maximized = deferred<boolean>();
+    const restored = deferred<boolean>();
+    let onResize: (() => void) | undefined;
+    const onChange = vi.fn();
+    const source: WindowMaximizedSource = {
+      isMaximized: vi.fn()
+        .mockReturnValueOnce(initial.promise)
+        .mockReturnValueOnce(maximized.promise)
+        .mockReturnValueOnce(restored.promise)
+        .mockResolvedValue(false),
+      listenResized: vi.fn(async listener => {
+        onResize = listener;
+        return vi.fn();
+      }),
+    };
+
+    const dispose = subscribeToWindowMaximized(source, onChange);
+    await vi.waitFor(() => expect(source.isMaximized).toHaveBeenCalledTimes(2));
+    onResize?.();
+    expect(source.isMaximized).toHaveBeenCalledTimes(3);
+
+    restored.resolve(false);
+    maximized.resolve(true);
+    initial.resolve(false);
+    await vi.waitFor(() => expect(onChange).toHaveBeenCalledWith(false));
+
+    expect(onChange).toHaveBeenCalledTimes(1);
+    dispose();
+  });
+
+  it('disposes a listener that finishes registering after unmount and ignores late state', async () => {
+    const registration = deferred<WindowStateDisposer>();
+    const state = deferred<boolean>();
+    const disposer = vi.fn();
+    const onChange = vi.fn();
+    const source: WindowMaximizedSource = {
+      isMaximized: () => state.promise,
+      listenResized: () => registration.promise,
+    };
+
+    const dispose = subscribeToWindowMaximized(source, onChange);
+    dispose();
+    registration.resolve(disposer);
+    state.resolve(true);
+
+    await vi.waitFor(() => expect(disposer).toHaveBeenCalledOnce());
+    expect(onChange).not.toHaveBeenCalled();
+  });
+
+  it('survives synchronous native state and listener failures', () => {
+    const source: WindowMaximizedSource = {
+      isMaximized: () => { throw new Error('state unavailable'); },
+      listenResized: () => { throw new Error('listener unavailable'); },
+    };
+
+    expect(() => subscribeToWindowMaximized(source, vi.fn())()).not.toThrow();
+  });
+});

src/utils/windowMaximized.ts

@@ -0,0 +1,91 @@
+import { getCurrentWindow } from '@tauri-apps/api/window';
+import { useEffect, useMemo, useState } from 'react';
+
+type WindowStateDisposer = () => void | Promise<void>;
+
+export type WindowMaximizedSource = {
+  isMaximized: () => boolean | Promise<boolean>;
+  listenResized: (listener: () => void) => Promise<WindowStateDisposer>;
+};
+
+const createCurrentWindowMaximizedSource = (): WindowMaximizedSource => {
+  const currentWindow = getCurrentWindow();
+  return {
+    isMaximized: () => currentWindow.isMaximized(),
+    listenResized: listener => currentWindow.onResized(listener),
+  };
+};
+
+const safelyDispose = (disposer: WindowStateDisposer | undefined) => {
+  if (typeof disposer !== 'function') return;
+  try {
+    void Promise.resolve(disposer()).catch(() => undefined);
+  } catch {
+    // Window teardown remains best-effort if the native listener is already gone.
+  }
+};
+
+export const subscribeToWindowMaximized = (
+  source: WindowMaximizedSource,
+  onChange: (maximized: boolean) => void,
+): (() => void) => {
+  let disposed = false;
+  let readGeneration = 0;
+  let lastPublished: boolean | undefined;
+  let listenerDisposer: WindowStateDisposer | undefined;
+
+  const refresh = () => {
+    const generation = ++readGeneration;
+    let stateRead: boolean | Promise<boolean>;
+    try {
+      stateRead = source.isMaximized();
+    } catch {
+      return;
+    }
+    void Promise.resolve(stateRead)
+      .then(maximized => {
+        if (disposed || generation !== readGeneration || maximized === lastPublished) return;
+        lastPublished = maximized;
+        onChange(maximized);
+      })
+      .catch(() => undefined);
+  };
+
+  let registration: Promise<WindowStateDisposer>;
+  try {
+    registration = Promise.resolve(source.listenResized(refresh));
+  } catch {
+    registration = Promise.reject();
+  }
+  void registration
+    .then(disposer => {
+      if (disposed) safelyDispose(disposer);
+      else if (typeof disposer === 'function') listenerDisposer = disposer;
+      // Close the registration gap with an authoritative post-listener read.
+      if (!disposed) refresh();
+    })
+    .catch(() => undefined);
+
+  // Do not delay the first state read behind asynchronous listener registration.
+  refresh();
+
+  return () => {
+    if (disposed) return;
+    disposed = true;
+    readGeneration += 1;
+    safelyDispose(listenerDisposer);
+    listenerDisposer = undefined;
+  };
+};
+
+export const useWindowMaximizedState = (providedSource?: WindowMaximizedSource): boolean => {
+  const source = useMemo(
+    () => providedSource ?? createCurrentWindowMaximizedSource(),
+    [providedSource],
+  );
+  const [maximized, setMaximized] = useState(false);
+
+  useEffect(() => subscribeToWindowMaximized(source, setMaximized), [source]);
+
+  return maximized;
+};