windisplay: Fix parented child window being offset

This would happen if the undecorated flag is not set; it would receive an additional offset equal to the size of the window decoration.

Fixes regression presumably caused by a968caf1d207023b2820cf4c31ae3e7324ffb218

panda/src/windisplay/winGraphicsWindow.cxx

@@ -298,26 +298,28 @@ set_properties_now(WindowProperties &properties) {
     // When switching undecorated mode, Windows will keep the window at the
     // current outer size, whereas we want to keep it with the configured
     // inner size.  Store the current size and origin.
-    LPoint2i top_left = _properties.get_origin();
-    LPoint2i bottom_right = top_left + _properties.get_size();
-
-    DWORD window_style = make_style(_properties);
-    SetWindowLong(_hWnd, GWL_STYLE, window_style);
-
-    // Now calculate the proper size and origin with the new window style.
-    RECT view_rect;
-    SetRect(&view_rect, top_left[0], top_left[1],
-            bottom_right[0], bottom_right[1]);
-    WINDOWINFO wi;
-    GetWindowInfo(_hWnd, &wi);
-    AdjustWindowRectEx(&view_rect, wi.dwStyle, FALSE, wi.dwExStyle);
-
-    // We need to call this to ensure that the style change takes effect.
-    SetWindowPos(_hWnd, HWND_NOTOPMOST, view_rect.left, view_rect.top,
-                 view_rect.right - view_rect.left,
-                 view_rect.bottom - view_rect.top,
-                 SWP_NOZORDER | SWP_NOACTIVATE | SWP_FRAMECHANGED |
-                 SWP_NOSENDCHANGING | SWP_SHOWWINDOW);
+    if (_parent_window_handle == nullptr) {
+      LPoint2i top_left = _properties.get_origin();
+      LPoint2i bottom_right = top_left + _properties.get_size();
+
+      DWORD window_style = make_style(_properties);
+      SetWindowLong(_hWnd, GWL_STYLE, window_style);
+
+      // Now calculate the proper size and origin with the new window style.
+      RECT view_rect;
+      SetRect(&view_rect, top_left[0], top_left[1],
+              bottom_right[0], bottom_right[1]);
+      WINDOWINFO wi;
+      GetWindowInfo(_hWnd, &wi);
+      AdjustWindowRectEx(&view_rect, wi.dwStyle, FALSE, wi.dwExStyle);
+
+      // We need to call this to ensure that the style change takes effect.
+      SetWindowPos(_hWnd, HWND_NOTOPMOST, view_rect.left, view_rect.top,
+                   view_rect.right - view_rect.left,
+                   view_rect.bottom - view_rect.top,
+                   SWP_NOZORDER | SWP_NOACTIVATE | SWP_FRAMECHANGED |
+                   SWP_NOSENDCHANGING | SWP_SHOWWINDOW);
+    }
   }
 
   if (properties.has_title()) {