fix(windows): fix double titlebar on Windows 10 when TitleBarStyle is hidden

When `titleBarStyle` is set to `TitleBarStyle.hidden` on Windows 10, adding a 1px top offset in `WM_NCCALCSIZE` leaves a non-client margin at the top of the window. On Windows 10, DWM interprets this non-client margin as an active caption area and forces rendering of the system default title bar alongside the Flutter custom title bar.
Removing the extra top offset allows the client area to cover the top edge completely, preventing DWM from drawing the native title bar.

Author: axel10

packages/window_manager/windows/window_manager_plugin.cpp

@@ -168,9 +168,9 @@ std::optional<LRESULT> WindowManagerPlugin::HandleWindowProc(HWND hWnd,
         adjustNCCALCSIZE(hWnd, reinterpret_cast<NCCALCSIZE_PARAMS*>(lParam));
       } else {
         NCCALCSIZE_PARAMS* sz = reinterpret_cast<NCCALCSIZE_PARAMS*>(lParam);
-        // on windows 10, if set to 0, there's a white line at the top
-        // of the app and I've yet to find a way to remove that.
-        sz->rgrc[0].top += IsWindows11OrGreater() ? 0 : 1;
+        // Do not add top offset on Windows 10, otherwise DWM interprets the non-zero
+        // non-client top margin as having a native caption and renders the system title bar.
+        sz->rgrc[0].top += 0;
         // The following lines are required for resizing the window.
         // https://github.com/leanflutter/window_manager/issues/483
         sz->rgrc[0].right -= 8;