spec/fixtures/pages/webview-devtools.html

<!DOCTYPE html>
<html>
  <head>
    <meta charset="utf-8">
  </head>
  <body>
    <webview nodeintegration src="./a.html"></webview>
    <script>
        var wv = document.querySelector('webview')
        wv.addEventListener('dom-ready', () => {
          const { remote } = require('electron')
          const webContents = remote.webContents.fromId(wv.getWebContentsId())
          webContents.on('devtools-opened', function () {
            var showPanelIntevalId = setInterval(function () {
              if (webContents.devToolsWebContents) {
                webContents.devToolsWebContents.executeJavaScript('(' + (function () {
                  var lastPanelId = UI.inspectorView._tabbedPane._tabs.peekLast().id
                  UI.inspectorView.showPanel(lastPanelId)
                }).toString() + ')()')
              } else {
                clearInterval(showPanelIntevalId)
              }
            }, 100)
          })

          wv.openDevTools()
        })
      </script>
    </script>
  </body>
</html>