Webpack/karma always reports 50% branch usage for all files

Question

I am trying to add code coverage to my web project and found that, no matter what I do, my coverage for all files is always 100% for statements, functions, lines, but branches are always 50% (it lies). And that was progress since adding karma-coverage-istanbul-instrumenter. Prior to that addition, it was always 0%.

I created a stand-alone project to demonstrate the issue (deleting karma-coverage-istanbul-instrumenter in karma.conf.js will return it to 0% coverage). It has one file, so arguably, not a great demonstration, but test and branch coverage is definitely 100%.

I did do some debugging in karma, and I found that it looks like when the result event fires, every file has exactly the same coverage. It's unclear which plugin is the issue. I did switch over to jasmine to see if it was "mocha" related, but it's not.

I'm using [email protected], [email protected] [email protected], and the latest of everything.

My karma.config.js file is as follows:

const path = require('path');
const webpackConfig = require('./webpack.config.js');

module.exports = function(config) {
    config.set({
        restartOnFileChange: true,
        logLevel: config.LOG_DEBUG,
        colors: true,
        singleRun: true,

        browsers: [ 'MyHeadlessChrome' ],

        customLaunchers: {
            MyHeadlessChrome: {
                base: 'ChromeHeadless',
                flags: [ '--no-sandbox' ]
            }
        },

        frameworks: [
            'webpack',
            'mocha',
            'chai'
        ],

        plugins: [
            'karma-webpack',
            'karma-mocha',
            'karma-chai',
            'karma-chrome-launcher',
            'karma-sourcemap-loader',
            'karma-mocha-reporter',
            'karma-coverage-istanbul-reporter',
            'karma-coverage-istanbul-instrumenter'
        ],

        files: [
            'test/*.spec.js',
            'src/*.js'
        ],

        preprocessors: {
            'test/*.spec.js': [ 'webpack' ],
            'src/*.js': [ 'webpack', 'sourcemap', 'karma-coverage-istanbul-instrumenter' ]
        },

        reporters: [ 'mocha', 'coverage-istanbul' ],

        coverageIstanbulReporter: {
            reports: [ 'html', 'lcov', 'text-summary' ],
            dir: path.join(__dirname, 'coverage'),
            fixWebpackSourcePaths: false,
            esModules: false,
            // enforce percentage thresholds
            // anything under these percentages will cause karma to fail
            // with an exit code of 1 if not running in watch mode
            thresholds: {
                    // set to `true` to not fail the test command when thresholds are not met
                    emitWarning: true,
                    global: { // thresholds for all files
                            statements: 94.09,
                            branches: 89.92,
                            functions: 92.67,
                            lines: 94.33
                    }
            }
        },

        webpack: webpackConfig
    });
}