If I understand correctly, you'd probably get what you want (a titleBar-less window) by changing the BaseWindowConstructorOptions preferences to this:

mainWindow = new BrowserWindow({
  height: 600,
  width: 1000,
  titleBarStyle: "hidden", // this hides the title bar
  webPreferences: {
    preload: MAIN_WINDOW_PRELOAD_WEBPACK_ENTRY,
  },
});

There's more options here if you'd like to check them out under "titleBarStyle": https://www.electronjs.org/docs/latest/api/structures/base-window-options

Best, Peter.