diff --git a/browser-entry.js b/browser-entry.js
index cdcb56b6fb..dee3789231 100644
--- a/browser-entry.js
+++ b/browser-entry.js
@@ -210,8 +210,8 @@ Mocha.process = process;
  * Expose mocha.
  */
 
-mocha.Mocha = Mocha;
-mocha.mocha = mocha;
+global.Mocha = Mocha;
+global.mocha = mocha;
 
 // this allows test/acceptance/required-tokens.js to pass; thus,
 // you can now do `const describe = require('mocha').describe` in a
diff --git a/lib/hook.js b/lib/hook.js
index 6c12c02bb8..8091e5d5da 100644
--- a/lib/hook.js
+++ b/lib/hook.js
@@ -1,7 +1,7 @@
 'use strict';
 
 var Runnable = require('./runnable');
-var inherits = require('./utils').inherits;
+const {inherits, constants} = require('./utils');
 
 /**
  * Expose `Hook`.
@@ -63,16 +63,21 @@ Hook.prototype.serialize = function serialize() {
   return {
     $$isPending: this.isPending(),
     $$titlePath: this.titlePath(),
-    ctx: {
-      currentTest: {
-        title: this.ctx && this.ctx.currentTest && this.ctx.currentTest.title
-      }
-    },
+    ctx:
+      this.ctx && this.ctx.currentTest
+        ? {
+            currentTest: {
+              title: this.ctx.currentTest.title,
+              [constants.MOCHA_ID_PROP_NAME]: this.ctx.currentTest.id
+            }
+          }
+        : {},
     parent: {
-      root: this.parent.root,
-      title: this.parent.title
+      [constants.MOCHA_ID_PROP_NAME]: this.parent.id
     },
     title: this.title,
-    type: this.type
+    type: this.type,
+    [constants.MOCHA_ID_PROP_NAME]: this.id,
+    __mocha_partial__: true
   };
 };
diff --git a/lib/mocha.js b/lib/mocha.js
index dc18b585a5..2c2eefd91f 100644
--- a/lib/mocha.js
+++ b/lib/mocha.js
@@ -128,8 +128,8 @@ exports.Test = require('./test');
  * suite with
  * @param {boolean} [options.isWorker] - Should be `true` if `Mocha` process is running in a worker process.
  */
-function Mocha(options) {
-  options = utils.assign({}, mocharc, options || {});
+function Mocha(options = {}) {
+  options = {...mocharc, ...options};
   this.files = [];
   this.options = options;
   // root suite
diff --git a/lib/nodejs/parallel-buffered-runner.js b/lib/nodejs/parallel-buffered-runner.js
index ee8635ab98..65cc415ac1 100644
--- a/lib/nodejs/parallel-buffered-runner.js
+++ b/lib/nodejs/parallel-buffered-runner.js
@@ -12,7 +12,8 @@ const {EVENT_RUN_BEGIN, EVENT_RUN_END} = Runner.constants;
 const debug = require('debug')('mocha:parallel:parallel-buffered-runner');
 const {BufferedWorkerPool} = require('./buffered-worker-pool');
 const {setInterval, clearInterval} = global;
-const {createMap} = require('../utils');
+const {createMap, constants} = require('../utils');
+const {MOCHA_ID_PROP_NAME} = constants;
 
 /**
  * Outputs a debug statement with worker stats
@@ -76,6 +77,9 @@ class ParallelBufferedRunner extends Runner {
       }
     });
 
+    this._linkPartialObjects = false;
+    this._linkedObjectMap = new Map();
+
     this.once(Runner.constants.EVENT_RUN_END, () => {
       this._state = COMPLETE;
     });
@@ -88,10 +92,60 @@ class ParallelBufferedRunner extends Runner {
    * @returns {FileRunner} Mapping function
    */
   _createFileRunner(pool, options) {
+    /**
+     * Emits event and sets `BAILING` state, if necessary.
+     * @param {Object} event - Event having `eventName`, maybe `data` and maybe `error`
+     * @param {number} failureCount - Failure count
+     */
+    const emitEvent = (event, failureCount) => {
+      this.emit(event.eventName, event.data, event.error);
+      if (
+        this._state !== BAILING &&
+        event.data &&
+        event.data._bail &&
+        (failureCount || event.error)
+      ) {
+        debug('run(): nonzero failure count & found bail flag');
+        // we need to let the events complete for this file, as the worker
+        // should run any cleanup hooks
+        this._state = BAILING;
+      }
+    };
+
+    /**
+     * Given an event, recursively find any objects in its data that have ID's, and create object references to already-seen objects.
+     * @param {Object} event - Event having `eventName`, maybe `data` and maybe `error`
+     */
+    const linkEvent = event => {
+      const stack = [{parent: event, prop: 'data'}];
+      while (stack.length) {
+        const {parent, prop} = stack.pop();
+        const obj = parent[prop];
+        let newObj;
+        if (obj && typeof obj === 'object' && obj[MOCHA_ID_PROP_NAME]) {
+          const id = obj[MOCHA_ID_PROP_NAME];
+          newObj = this._linkedObjectMap.has(id)
+            ? Object.assign(this._linkedObjectMap.get(id), obj)
+            : obj;
+          this._linkedObjectMap.set(id, newObj);
+          parent[prop] = newObj;
+        } else {
+          newObj = obj;
+        }
+        Object.keys(newObj).forEach(key => {
+          const value = obj[key];
+          if (value && typeof value === 'object' && value[MOCHA_ID_PROP_NAME]) {
+            stack.push({obj: value, parent: newObj, prop: key});
+          }
+        });
+      }
+    };
+
     return async file => {
       debug('run(): enqueueing test file %s', file);
       try {
         const {failureCount, events} = await pool.run(file, options);
+
         if (this._state === BAILED) {
           // short-circuit after a graceful bail. if this happens,
           // some other worker has bailed.
@@ -107,20 +161,18 @@ class ParallelBufferedRunner extends Runner {
         );
         this.failures += failureCount; // can this ever be non-numeric?
         let event = events.shift();
-        while (event) {
-          this.emit(event.eventName, event.data, event.error);
-          if (
-            this._state !== BAILING &&
-            event.data &&
-            event.data._bail &&
-            (failureCount || event.error)
-          ) {
-            debug('run(): nonzero failure count & found bail flag');
-            // we need to let the events complete for this file, as the worker
-            // should run any cleanup hooks
-            this._state = BAILING;
+
+        if (this._linkPartialObjects) {
+          while (event) {
+            linkEvent(event);
+            emitEvent(event, failureCount);
+            event = events.shift();
+          }
+        } else {
+          while (event) {
+            emitEvent(event, failureCount);
+            event = events.shift();
           }
-          event = events.shift();
         }
         if (this._state === BAILING) {
           debug('run(): terminating pool due to "bail" flag');
@@ -275,6 +327,32 @@ class ParallelBufferedRunner extends Runner {
     })();
     return this;
   }
+
+  /**
+   * Toggle partial object linking behavior; used for building object references from
+   * unique ID's.
+   * @param {boolean} [value] - If `true`, enable partial object linking, otherwise disable
+   * @returns {Runner}
+   * @chainable
+   * @public
+   * @example
+   * // this reporter needs proper object references when run in parallel mode
+   * class MyReporter() {
+   *   constructor(runner) {
+   *     this.runner.linkPartialObjects(true)
+   *       .on(EVENT_SUITE_BEGIN, suite => {
+             // this Suite may be the same object...
+  *       })
+  *       .on(EVENT_TEST_BEGIN, test => {
+  *         // ...as the `test.parent` property
+  *       });
+  *   }
+  * }
+  */
+  linkPartialObjects(value) {
+    this._linkPartialObjects = Boolean(value);
+    return super.linkPartialObjects(value);
+  }
 }
 
 module.exports = ParallelBufferedRunner;
diff --git a/lib/runnable.js b/lib/runnable.js
index 023481dd69..e65b4c4638 100644
--- a/lib/runnable.js
+++ b/lib/runnable.js
@@ -38,6 +38,12 @@ function Runnable(title, fn) {
   this._timeout = 2000;
   this._slow = 75;
   this._retries = -1;
+  utils.assignNewMochaID(this);
+  Object.defineProperty(this, 'id', {
+    get() {
+      return utils.getMochaID(this);
+    }
+  });
   this.reset();
 }
 
diff --git a/lib/runner.js b/lib/runner.js
index cd91173e10..c8b5d24aa3 100644
--- a/lib/runner.js
+++ b/lib/runner.js
@@ -1043,6 +1043,33 @@ Runner.prototype.run = function(fn, opts) {
   return this;
 };
 
+/**
+ * Toggle partial object linking behavior; used for building object references from
+ * unique ID's. Does nothing in serial mode, because the object references already exist.
+ * Subclasses can implement this (e.g., `ParallelBufferedRunner`)
+ * @abstract
+ * @param {boolean} [value] - If `true`, enable partial object linking, otherwise disable
+ * @returns {Runner}
+ * @chainable
+ * @public
+ * @example
+ * // this reporter needs proper object references when run in parallel mode
+ * class MyReporter() {
+ *   constructor(runner) {
+ *     this.runner.linkPartialObjects(true)
+ *       .on(EVENT_SUITE_BEGIN, suite => {
+           // this Suite may be the same object...
+ *       })
+ *       .on(EVENT_TEST_BEGIN, test => {
+ *         // ...as the `test.parent` property
+ *       });
+ *   }
+ * }
+ */
+Runner.prototype.linkPartialObjects = function(value) {
+  return this;
+};
+
 /**
  * Cleanly abort execution.
  *
diff --git a/lib/suite.js b/lib/suite.js
index e9d45d94c4..320582f611 100644
--- a/lib/suite.js
+++ b/lib/suite.js
@@ -13,6 +13,8 @@ var milliseconds = require('ms');
 var errors = require('./errors');
 var createInvalidArgumentTypeError = errors.createInvalidArgumentTypeError;
 
+const {MOCHA_ID_PROP_NAME} = utils.constants;
+
 /**
  * Expose `Suite`.
  */
@@ -74,6 +76,14 @@ function Suite(title, parentContext, isRoot) {
   this._bail = false;
   this._onlyTests = [];
   this._onlySuites = [];
+  utils.assignNewMochaID(this);
+
+  Object.defineProperty(this, 'id', {
+    get() {
+      return utils.getMochaID(this);
+    }
+  });
+
   this.reset();
 
   this.on('newListener', function(event) {
@@ -579,7 +589,9 @@ Suite.prototype.serialize = function serialize() {
     $$fullTitle: this.fullTitle(),
     $$isPending: this.isPending(),
     root: this.root,
-    title: this.title
+    title: this.title,
+    id: this.id,
+    parent: this.parent ? {[MOCHA_ID_PROP_NAME]: this.parent.id} : null
   };
 };
 
diff --git a/lib/test.js b/lib/test.js
index 3fb3e57a4e..f2be51b4f4 100644
--- a/lib/test.js
+++ b/lib/test.js
@@ -5,6 +5,8 @@ var errors = require('./errors');
 var createInvalidArgumentTypeError = errors.createInvalidArgumentTypeError;
 var isString = utils.isString;
 
+const {MOCHA_ID_PROP_NAME} = utils.constants;
+
 module.exports = Test;
 
 /**
@@ -98,12 +100,14 @@ Test.prototype.serialize = function serialize() {
     duration: this.duration,
     err: this.err,
     parent: {
-      $$fullTitle: this.parent.fullTitle()
+      $$fullTitle: this.parent.fullTitle(),
+      [MOCHA_ID_PROP_NAME]: this.parent.id
     },
     speed: this.speed,
     state: this.state,
     title: this.title,
     type: this.type,
-    file: this.file
+    file: this.file,
+    [MOCHA_ID_PROP_NAME]: this.id
   };
 };
diff --git a/lib/utils.js b/lib/utils.js
index 00d6fcc938..0fd7ca2c4d 100644
--- a/lib/utils.js
+++ b/lib/utils.js
@@ -9,11 +9,12 @@
  * Module dependencies.
  */
 
+const {nanoid} = require('nanoid/non-secure');
 var path = require('path');
 var util = require('util');
 var he = require('he');
 
-var assign = (exports.assign = require('object.assign').getPolyfill());
+const MOCHA_ID_PROP_NAME = '__mocha_id__';
 
 /**
  * Inherit the prototype methods from one constructor into another.
@@ -575,7 +576,7 @@ exports.noop = function() {};
  * @returns {Object} An object with no prototype, having `...obj` properties
  */
 exports.createMap = function(obj) {
-  return assign.apply(
+  return Object.assign.apply(
     null,
     [Object.create(null)].concat(Array.prototype.slice.call(arguments))
   );
@@ -645,3 +646,31 @@ exports.cwd = function cwd() {
 exports.isBrowser = function isBrowser() {
   return Boolean(process.browser);
 };
+
+exports.constants = exports.defineConstants({
+  MOCHA_ID_PROP_NAME
+});
+
+/**
+ * Creates a new unique identifier
+ * @returns {string} Unique identifier
+ */
+exports.uniqueID = () => nanoid();
+
+exports.assignNewMochaID = obj => {
+  const id = exports.uniqueID();
+  Object.defineProperty(obj, MOCHA_ID_PROP_NAME, {
+    get() {
+      return id;
+    }
+  });
+  return obj;
+};
+
+/**
+ * Retrieves a Mocha ID from an object, if present.
+ * @param {*} [obj] - Object
+ * @returns {string|void}
+ */
+exports.getMochaID = obj =>
+  obj && typeof obj === 'object' ? obj[MOCHA_ID_PROP_NAME] : undefined;
diff --git a/package-lock.json b/package-lock.json
index 328c2a81d2..971a4e1d35 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -3488,13 +3488,53 @@
       }
     },
     "@rollup/plugin-babel": {
-      "version": "5.0.4",
-      "resolved": "https://registry.npmjs.org/@rollup/plugin-babel/-/plugin-babel-5.0.4.tgz",
-      "integrity": "sha512-MBtNoi5gqBEbqy1gE9jZBfPsi10kbuK2CEu9bx53nk1Z3ATRvBOoZ/GsbhXOeVbS76xXi/DeYM+vYX6EGIDv9A==",
+      "version": "5.2.0",
+      "resolved": "https://registry.npmjs.org/@rollup/plugin-babel/-/plugin-babel-5.2.0.tgz",
+      "integrity": "sha512-CPABsajaKjINgBQ3it+yMnfVO3ibsrMBxRzbUOUw2cL1hsZJ7aogU8mgglQm3S2hHJgjnAmxPz0Rq7DVdmHsTw==",
       "dev": true,
       "requires": {
-        "@babel/helper-module-imports": "^7.7.4",
-        "@rollup/pluginutils": "^3.0.8"
+        "@babel/helper-module-imports": "^7.10.4",
+        "@rollup/pluginutils": "^3.1.0"
+      },
+      "dependencies": {
+        "@babel/helper-module-imports": {
+          "version": "7.10.4",
+          "resolved": "https://registry.npmjs.org/@babel/helper-module-imports/-/helper-module-imports-7.10.4.tgz",
+          "integrity": "sha512-nEQJHqYavI217oD9+s5MUBzk6x1IlvoS9WTPfgG43CbMEeStE0v+r+TucWdx8KFGowPGvyOkDT9+7DHedIDnVw==",
+          "dev": true,
+          "requires": {
+            "@babel/types": "^7.10.4"
+          }
+        },
+        "@babel/helper-validator-identifier": {
+          "version": "7.10.4",
+          "resolved": "https://registry.npmjs.org/@babel/helper-validator-identifier/-/helper-validator-identifier-7.10.4.tgz",
+          "integrity": "sha512-3U9y+43hz7ZM+rzG24Qe2mufW5KhvFg/NhnNph+i9mgCtdTCtMJuI1TMkrIUiK7Ix4PYlRF9I5dhqaLYA/ADXw==",
+          "dev": true
+        },
+        "@babel/types": {
+          "version": "7.11.0",
+          "resolved": "https://registry.npmjs.org/@babel/types/-/types-7.11.0.tgz",
+          "integrity": "sha512-O53yME4ZZI0jO1EVGtF1ePGl0LHirG4P1ibcD80XyzZcKhcMFeCXmh4Xb1ifGBIV233Qg12x4rBfQgA+tmOukA==",
+          "dev": true,
+          "requires": {
+            "@babel/helper-validator-identifier": "^7.10.4",
+            "lodash": "^4.17.19",
+            "to-fast-properties": "^2.0.0"
+          }
+        },
+        "lodash": {
+          "version": "4.17.20",
+          "resolved": "https://registry.npmjs.org/lodash/-/lodash-4.17.20.tgz",
+          "integrity": "sha512-PlhdFcillOINfeV7Ni6oF1TAEayyZBoZ8bcshTHqOYJYlrqzRK5hagpagky5o4HfCzzd1TRkXPMFq6cKk9rGmA==",
+          "dev": true
+        },
+        "to-fast-properties": {
+          "version": "2.0.0",
+          "resolved": "https://registry.npmjs.org/to-fast-properties/-/to-fast-properties-2.0.0.tgz",
+          "integrity": "sha1-3F5pjL0HkmW8c+A3doGk5Og/YW4=",
+          "dev": true
+        }
       }
     },
     "@rollup/plugin-commonjs": {
@@ -7367,6 +7407,12 @@
         "xdg-basedir": "^3.0.0"
       },
       "dependencies": {
+        "crypto-random-string": {
+          "version": "1.0.0",
+          "resolved": "https://registry.npmjs.org/crypto-random-string/-/crypto-random-string-1.0.0.tgz",
+          "integrity": "sha1-ojD2T1aDEOFJgAmUB5DsmVRbyn4=",
+          "dev": true
+        },
         "dot-prop": {
           "version": "4.2.0",
           "resolved": "https://registry.npmjs.org/dot-prop/-/dot-prop-4.2.0.tgz",
@@ -7381,6 +7427,15 @@
           "resolved": "https://registry.npmjs.org/is-obj/-/is-obj-1.0.1.tgz",
           "integrity": "sha1-PkcprB9f3gJc19g6iW2rn09n2w8=",
           "dev": true
+        },
+        "unique-string": {
+          "version": "1.0.0",
+          "resolved": "https://registry.npmjs.org/unique-string/-/unique-string-1.0.0.tgz",
+          "integrity": "sha1-nhBXzKhRq7kzmPizOuGHuZyuwRo=",
+          "dev": true,
+          "requires": {
+            "crypto-random-string": "^1.0.0"
+          }
         }
       }
     },
@@ -7675,12 +7730,6 @@
         "randomfill": "^1.0.3"
       }
     },
-    "crypto-random-string": {
-      "version": "1.0.0",
-      "resolved": "https://registry.npmjs.org/crypto-random-string/-/crypto-random-string-1.0.0.tgz",
-      "integrity": "sha1-ojD2T1aDEOFJgAmUB5DsmVRbyn4=",
-      "dev": true
-    },
     "css-color-names": {
       "version": "0.0.4",
       "resolved": "https://registry.npmjs.org/css-color-names/-/css-color-names-0.0.4.tgz",
@@ -15237,10 +15286,9 @@
       "dev": true
     },
     "nanoid": {
-      "version": "2.1.11",
-      "resolved": "https://registry.npmjs.org/nanoid/-/nanoid-2.1.11.tgz",
-      "integrity": "sha512-s/snB+WGm6uwi0WjsZdaVcuf3KJXlfGl2LcxgwkEwJF0D/BWzVWAZW/XY4bFaiR7s0Jk3FPvlnepg1H1b1UwlA==",
-      "dev": true
+      "version": "3.1.12",
+      "resolved": "https://registry.npmjs.org/nanoid/-/nanoid-3.1.12.tgz",
+      "integrity": "sha512-1qstj9z5+x491jfiC4Nelk+f8XBad7LN20PmyWINJEMRSf3wcAjAWysw1qaA8z6NSKe2sjq1hRSDpBH5paCb6A=="
     },
     "nanomatch": {
       "version": "1.2.13",
@@ -18981,6 +19029,12 @@
             "p-locate": "^4.1.0"
           }
         },
+        "nanoid": {
+          "version": "2.1.11",
+          "resolved": "https://registry.npmjs.org/nanoid/-/nanoid-2.1.11.tgz",
+          "integrity": "sha512-s/snB+WGm6uwi0WjsZdaVcuf3KJXlfGl2LcxgwkEwJF0D/BWzVWAZW/XY4bFaiR7s0Jk3FPvlnepg1H1b1UwlA==",
+          "dev": true
+        },
         "p-locate": {
           "version": "4.1.0",
           "resolved": "https://registry.npmjs.org/p-locate/-/p-locate-4.1.0.tgz",
@@ -21656,15 +21710,6 @@
       "integrity": "sha1-/+3ks2slKQaW5uFl1KWe25mOawI=",
       "dev": true
     },
-    "unique-string": {
-      "version": "1.0.0",
-      "resolved": "https://registry.npmjs.org/unique-string/-/unique-string-1.0.0.tgz",
-      "integrity": "sha1-nhBXzKhRq7kzmPizOuGHuZyuwRo=",
-      "dev": true,
-      "requires": {
-        "crypto-random-string": "^1.0.0"
-      }
-    },
     "unist-util-is": {
       "version": "3.0.0",
       "resolved": "https://registry.npmjs.org/unist-util-is/-/unist-util-is-3.0.0.tgz",
@@ -21944,9 +21989,9 @@
       "dev": true
     },
     "uuid": {
-      "version": "8.0.0",
-      "resolved": "https://registry.npmjs.org/uuid/-/uuid-8.0.0.tgz",
-      "integrity": "sha512-jOXGuXZAWdsTH7eZLtyXMqUb9EcWMGZNbL9YcGBJl4MH4nrxHmZJhEHvyLFrkxo+28uLb/NYRcStH48fnD0Vzw==",
+      "version": "8.3.0",
+      "resolved": "https://registry.npmjs.org/uuid/-/uuid-8.3.0.tgz",
+      "integrity": "sha512-fX6Z5o4m6XsXBdli9g7DtWgAx+osMsRRZFKma1mIUsLCz6vRvv+pz5VNbyu9UEDzpMWulZfvpgb/cmDXVulYFQ==",
       "dev": true
     },
     "v8-compile-cache": {
diff --git a/package.json b/package.json
index 6f88bd7033..08901adc92 100644
--- a/package.json
+++ b/package.json
@@ -65,7 +65,7 @@
     "log-symbols": "3.0.0",
     "minimatch": "3.0.4",
     "ms": "2.1.2",
-    "object.assign": "4.1.0",
+    "nanoid": "3.1.12",
     "promise.allsettled": "1.0.2",
     "serialize-javascript": "4.0.0",
     "strip-json-comments": "3.0.1",
diff --git a/rollup.config.js b/rollup.config.js
index e90bb5203e..4b4f3494e4 100644
--- a/rollup.config.js
+++ b/rollup.config.js
@@ -17,8 +17,7 @@ const config = {
     file: './mocha.js',
     format: 'umd',
     sourcemap: true,
-    name: 'mocha',
-    footer: 'window.Mocha = window.mocha.Mocha;'
+    name: 'mocha'
   },
   plugins: [
     json(),
@@ -38,7 +37,6 @@ const config = {
           '@babel/preset-env',
           {
             modules: false,
-            spec: true,
             useBuiltIns: 'usage',
             forceAllTransforms: true,
             corejs: {
diff --git a/test/node-unit/buffered-runner.spec.js b/test/node-unit/parallel-buffered-runner.spec.js
similarity index 83%
rename from test/node-unit/buffered-runner.spec.js
rename to test/node-unit/parallel-buffered-runner.spec.js
index ef308f0905..225388dea4 100644
--- a/test/node-unit/buffered-runner.spec.js
+++ b/test/node-unit/parallel-buffered-runner.spec.js
@@ -8,19 +8,18 @@ const {
   EVENT_SUITE_BEGIN
 } = require('../../lib/runner').constants;
 const rewiremock = require('rewiremock/node');
-const BUFFERED_RUNNER_PATH = require.resolve(
-  '../../lib/nodejs/parallel-buffered-runner.js'
-);
 const Suite = require('../../lib/suite');
 const Runner = require('../../lib/runner');
 const sinon = require('sinon');
+const {constants} = require('../../lib/utils');
+const {MOCHA_ID_PROP_NAME} = constants;
 
-describe('buffered-runner', function() {
-  describe('BufferedRunner', function() {
+describe('parallel-buffered-runner', function() {
+  describe('ParallelBufferedRunner', function() {
     let run;
     let BufferedWorkerPool;
     let terminate;
-    let BufferedRunner;
+    let ParallelBufferedRunner;
     let suite;
     let warn;
     let cpuCount;
@@ -40,20 +39,31 @@ describe('buffered-runner', function() {
           stats: sinon.stub().returns({})
         })
       };
-      BufferedRunner = rewiremock.proxy(BUFFERED_RUNNER_PATH, r => ({
-        '../../lib/nodejs/buffered-worker-pool': {
-          BufferedWorkerPool
-        },
-        os: {
-          cpus: sinon.stub().callsFake(() => new Array(cpuCount))
-        },
-        '../../lib/utils': r.with({warn}).callThrough()
-      }));
+      /**
+       * @type {ParallelBufferedRunner}
+       */
+      ParallelBufferedRunner = rewiremock.proxy(
+        () => require('../../lib/nodejs/parallel-buffered-runner'),
+        r => ({
+          '../../lib/nodejs/buffered-worker-pool': {
+            BufferedWorkerPool
+          },
+          os: {
+            cpus: sinon.stub().callsFake(() => new Array(cpuCount))
+          },
+          '../../lib/utils': r.with({warn}).callThrough()
+        })
+      );
     });
 
     describe('constructor', function() {
       it('should start in "IDLE" state', function() {
-        expect(new BufferedRunner(suite), 'to have property', '_state', 'IDLE');
+        expect(
+          new ParallelBufferedRunner(suite),
+          'to have property',
+          '_state',
+          'IDLE'
+        );
       });
     });
 
@@ -61,7 +71,7 @@ describe('buffered-runner', function() {
       let runner;
 
       beforeEach(function() {
-        runner = new BufferedRunner(suite);
+        runner = new ParallelBufferedRunner(suite);
       });
 
       describe('_state', function() {
@@ -81,7 +91,7 @@ describe('buffered-runner', function() {
       let runner;
 
       beforeEach(function() {
-        runner = new BufferedRunner(suite);
+        runner = new ParallelBufferedRunner(suite);
       });
 
       describe('EVENT_RUN_END', function() {
@@ -94,11 +104,11 @@ describe('buffered-runner', function() {
     });
 
     describe('instance method', function() {
-      describe('run', function() {
+      describe('run()', function() {
         let runner;
 
         beforeEach(function() {
-          runner = new BufferedRunner(suite);
+          runner = new ParallelBufferedRunner(suite);
         });
 
         // the purpose of this is to ensure that--despite using `Promise`s
@@ -119,6 +129,64 @@ describe('buffered-runner', function() {
           );
         });
 
+        describe('when instructed to link objects', function() {
+          beforeEach(function() {
+            runner.linkPartialObjects(true);
+          });
+
+          it('should create object references', function() {
+            const options = {};
+            const someSuite = {
+              title: 'some suite',
+              [MOCHA_ID_PROP_NAME]: 'bar'
+            };
+
+            run.withArgs('some-file.js', options).resolves({
+              failureCount: 0,
+              events: [
+                {
+                  eventName: EVENT_SUITE_END,
+                  data: someSuite
+                },
+                {
+                  eventName: EVENT_TEST_PASS,
+                  data: {
+                    title: 'some test',
+                    [MOCHA_ID_PROP_NAME]: 'foo',
+                    parent: {
+                      // this stub object points to someSuite with id 'bar'
+                      [MOCHA_ID_PROP_NAME]: 'bar'
+                    }
+                  }
+                },
+                {
+                  eventName: EVENT_SUITE_END,
+                  // ensure we are not passing the _same_ someSuite,
+                  // because we won't get the same one from the subprocess
+                  data: {...someSuite}
+                }
+              ]
+            });
+
+            return expect(
+              () =>
+                new Promise(resolve => {
+                  runner.run(resolve, {files: ['some-file.js'], options: {}});
+                }),
+              'to emit from',
+              runner,
+              EVENT_TEST_PASS,
+              {
+                title: 'some test',
+                [MOCHA_ID_PROP_NAME]: 'foo',
+                parent: expect
+                  .it('to be', someSuite)
+                  .and('to have property', 'title', 'some suite')
+              }
+            );
+          });
+        });
+
         describe('when a worker fails', function() {
           it('should recover', function(done) {
             const options = {};
@@ -546,6 +614,20 @@ describe('buffered-runner', function() {
           });
         });
       });
+
+      describe('linkPartialObjects()', function() {
+        let runner;
+
+        beforeEach(function() {
+          runner = new ParallelBufferedRunner(suite);
+        });
+
+        it('should return the runner', function() {
+          expect(runner.linkPartialObjects(), 'to be', runner);
+        });
+
+        // avoid testing implementation details; don't check _linkPartialObjects
+      });
     });
   });
 });
diff --git a/test/unit/mocha.spec.js b/test/unit/mocha.spec.js
index 75be0341c9..98d6082948 100644
--- a/test/unit/mocha.spec.js
+++ b/test/unit/mocha.spec.js
@@ -51,7 +51,8 @@ describe('Mocha', function() {
     sinon.stub(Mocha.reporters, 'base').returns({});
     sinon.stub(Mocha.reporters, 'spec').returns({});
 
-    runner = utils.assign(sinon.createStubInstance(EventEmitter), {
+    runner = {
+      ...sinon.createStubInstance(EventEmitter),
       run: sinon
         .stub()
         .callsArgAsync(0)
@@ -59,18 +60,19 @@ describe('Mocha', function() {
       globals: sinon.stub(),
       grep: sinon.stub(),
       dispose: sinon.stub()
-    });
+    };
     Runner = sinon.stub(Mocha, 'Runner').returns(runner);
     // the Runner constructor is the main export, and constants is a static prop.
     // we don't need the constants themselves, but the object cannot be undefined
     Runner.constants = {};
-    suite = utils.assign(sinon.createStubInstance(EventEmitter), {
+    suite = {
+      ...sinon.createStubInstance(EventEmitter),
       slow: sinon.stub(),
       timeout: sinon.stub(),
       bail: sinon.stub(),
       dispose: sinon.stub(),
       reset: sinon.stub()
-    });
+    };
     Suite = sinon.stub(Mocha, 'Suite').returns(suite);
     Suite.constants = {};
 
diff --git a/test/unit/runner.spec.js b/test/unit/runner.spec.js
index 730c40003d..a8d0f975ff 100644
--- a/test/unit/runner.spec.js
+++ b/test/unit/runner.spec.js
@@ -1158,4 +1158,10 @@ describe('Runner', function() {
       });
     });
   });
+
+  describe('linkPartialObjects', function() {
+    it('should return the Runner', function() {
+      expect(runner.linkPartialObjects(), 'to be', runner);
+    });
+  });
 });
diff --git a/test/unit/utils.spec.js b/test/unit/utils.spec.js
index 9fa6cc74e4..efd82f9ce4 100644
--- a/test/unit/utils.spec.js
+++ b/test/unit/utils.spec.js
@@ -742,4 +742,10 @@ describe('lib/utils', function() {
       expect(utils.slug('poppies & fritz'), 'to be', 'poppies-fritz');
     });
   });
+
+  describe('uniqueID()', function() {
+    it('should return a non-empty string', function() {
+      expect(utils.uniqueID(), 'to be a string').and('not to be empty');
+    });
+  });
 });