user32: The client rectangle is in screen coordinates for the initial WM_NCCALCSIZE.

dlls/user32/tests/win.c

@@ -3929,6 +3929,15 @@ static LRESULT CALLBACK winsizes_wnd_proc(HWND hwnd, UINT msg, WPARAM wp, LPARAM
             expected_rect.left, expected_rect.top, expected_rect.right, expected_rect.bottom );
         return DefWindowProc(hwnd, msg, wp, lp);
     }
+    case WM_NCCALCSIZE:
+    {
+        RECT rect, *r = (RECT *)lp;
+        GetWindowRect( hwnd, &rect );
+        ok( !memcmp( &rect, r, sizeof(rect) ),
+            "passed rect %d,%d-%d,%d doesn't match window rect %d,%d-%d,%d\n",
+            r->left, r->top, r->right, r->bottom, rect.left, rect.top, rect.right, rect.bottom );
+        return DefWindowProc(hwnd, msg, wp, lp);
+    }
     default:
         return DefWindowProc(hwnd, msg, wp, lp);
     }

dlls/user32/win.c

@@ -1139,11 +1139,18 @@ static HWND WIN_CreateWindowEx( CREATESTRUCTA *cs, LPCWSTR className, UINT flags
     if ((wndPtr = WIN_GetPtr(hwnd)))
     {
         /* yes, even if the CBT hook was called with HWND_TOP */
+        POINT pt;
         HWND insert_after = (wndPtr->dwStyle & WS_CHILD) ? HWND_BOTTOM : HWND_TOP;
         RECT window_rect = wndPtr->rectWindow;
         RECT client_rect = window_rect;
         WIN_ReleasePtr( wndPtr );
+
+        /* the rectangle is in screen coords for WM_NCCALCSIZE when wparam is FALSE */
+        pt.x = pt.y = 0;
+        MapWindowPoints( parent, 0, &pt, 1 );
+        OffsetRect( &client_rect, pt.x, pt.y );
         SendMessageW( hwnd, WM_NCCALCSIZE, FALSE, (LPARAM)&client_rect );
+        OffsetRect( &client_rect, -pt.x, -pt.y );
         set_window_pos( hwnd, insert_after, SWP_NOACTIVATE, &window_rect, &client_rect, NULL );
     }
     else return 0;