'devtools' protocol: Popup win closes itself: getWindowHandles() hangs, 100% CPU

Environment (please complete the following information):

• WebdriverIO version: 6.0.14
• Mode: WDIO Testrunner: ./node_modules/.bin/wdio
• If WDIO Testrunner, running sync/async: sync
• Node.js version: nodejs --version —> v12.16.2
• NPM version: npm --version —> 6.13.4
• Browser name and version: Chrome, Version 81.0.4044.92 (Official Build) (64-bit)
• Platform name and version: Ubuntu 16.04.6 LTS
• Additional wdio packages used (if applicable): My package.json:

{
  "author": "Magnus Lindberg",
  "name": "",
  "version": "0.0.1",
  "license": "",
  "engines": {
    "node": ">= 12.16.0"
  },
  "dependencies": {},
  "devDependencies": {
    "@wdio/cli": "^6.0.9",
    "@wdio/devtools-service": "^6.0.8",
    "@wdio/dot-reporter": "^6.0.8",
    "@wdio/firefox-profile-service": "^6.0.8",
    "@wdio/local-runner": "^6.0.9",
    "@wdio/mocha-framework": "^6.0.13",
    "@wdio/sync": "^6.0.8",
    "chromedriver": "^80.0.2",
    "http-server": "^0.11.1",
    "wdio-chromedriver-service": "^6.0.1"
  }
}

Config of WebdriverIO My wdio.conf.js: (it's just a default generated file, + path to the ... test.js)

exports.config = {
    runner: 'local',
    specs: [ './wdio-get-win-handles-self-closing-window.test.js' ],
    exclude: [],
    maxInstances: 10,
    capabilities: [{ maxInstances: 5, browserName: 'chrome' }],
    logLevel: 'trace', // trace debug info warn error silent
    bail: 0,
    baseUrl: 'http://localhost',
    waitforTimeout: 10000,
    connectionRetryTimeout: 90000,
    connectionRetryCount: 3,
    services: ['devtools'], // 'chromedriver' works though
    framework: 'mocha',
    reporters: ['dot'],
    mochaOpts: { ui: 'bdd', timeout: 60000 },
}

Describe the bug

A 2nd window opens (say, a login popup). WebdriverIO switches to this window. This 2nd window closes itself (say, after the user has logged in).

Thereaftter, the next call to browser.getWindowHandles() never returns — instead it pegs the CPU at 100%, and the node process won't exit, kill PID won't work, needs: kill -9 PID.

To Reproduce

Copy-paste into a new directory package.json, wdio.conf.js from above, plus the following *.html files and .js test file:

https://gist.github.com/kajmagnus/75f091bb12be830e81dcba246c2634da

Then: npm install. Then: ./node_modules/.bin/http-server -p8080 ./ — a static file server on port 8080. Then: ./node_modules/.bin/wdio wdio.conf.js — to run the test.

When the 2nd window closes, notice how the log messages from the while loop, stop abruptly — a request to browser.getWindowHandles() has somehow hanged.

Look at your CPU: A node process on my laptop now is 100%, needs kill -9.

Expected behavior

browser.getWindowHandles() should return 1 handle, not block.

Log

https://gist.github.com/kajmagnus/2b7f4e03f4adb06678d369ef5e97252b

Additional context

I might be one of the first people upgrading to WebdriverIO v6 and trying out the service: ['devtools']? (it was released just a few days or a week ago)