is the problem of resizadle: false. Comment out and fix the minimum and maximum width and height of the form

mainWindow = new BrowserWindow({
    width: 1240,
    height: 800,
    minWidth: 1240,
    minHeight: 800,
    maxWidth: 1240,
    maxHeight: 800,
    center: true
    // useContentSize: true
    // resizable: false
    // 
    // frame: false
  })
  // 
  // mainWindow.setResizable(false)
  // mainWindow.setMaximizable(false)

in addition, whether you set resizable: false, or mainWindow.setResizable (false), in the main thread or in the rendering process

import { remote } from 'electron'
remote.getCurrentWindow().setResizable(false)

.
and, if you keep the window maximize button, useContentSize: true, it's best to remove this, otherwise the form will change slightly when you click maximize.

I have the same problem, but my window height is getting smaller and smaller. This happens only when the Windows system is running and the main process sets the option that the window is not allowed to be resized.
mainWindow = new BrowserWindow ({

width: 340,
height: 550,
useContentSize: true,
resizable: false, // 
fullscreen: false,
fullscreenable: false,
webPreferences: {
  // devTools: false // 
}

})