I updated karma-webkit-launcher from 1.1.0 to 1.2.0 to get a resolution for #1 in my jQuery PR that I mentioned in #2 but now WebkitHeadless is no longer headless - a regular window appears.

You can test that easily on macOS by checking out my jQuery PR: jquery/jquery#5190 and invoking the following in the project directory:

npm install
npx grunt
BROWSERS=WebkitHeadless npx grunt karma:main