fix --parallel --watch; closes #4327

lib/cli/watch-run.js

@@ -36,11 +36,33 @@ exports.watchParallelRun = (
     watchFiles,
     watchIgnore,
     beforeRun({mocha}) {
-      mocha.files = collectFiles(fileCollectParams);
+      // I don't know why we're cloning the root suite.
+      const rootSuite = mocha.suite.clone();
+
+      // this `require` is needed because the require cache has been cleared.  the dynamic
+      // exports set via the below call to `mocha.ui()` won't work properly if a
+      // test depends on this module (see `required-tokens.spec.js`).
+      const Mocha = require('../mocha');
+
+      // ... and now that we've gotten a new module, we need to use it again due
+      // to `mocha.ui()` call
+      const newMocha = new Mocha(mocha.options);
+      // don't know why this is needed
+      newMocha.suite = rootSuite;
+      // nor this
+      newMocha.suite.ctx = new Context();
+
+      // reset the list of files
+      newMocha.files = collectFiles(fileCollectParams);
+
+      // because we've swapped out the root suite (see the `run` inner function
+      // in `createRerunner`), we need to call `mocha.ui()` again to set up the context/globals.
+      newMocha.ui(newMocha.options.ui);
+
       // in parallel mode, the main Mocha process doesn't actually load the
       // files. this flag prevents `mocha.run()` from autoloading.
-      mocha.lazyLoadFiles(true);
-      return mocha;
+      newMocha.lazyLoadFiles(true);
+      return newMocha;
     },
     afterRun({watcher}) {
       blastCache(watcher);

test/integration/options/watch.spec.js

@@ -31,6 +31,19 @@ describe('--watch', function() {
       });
     });
 
+    describe('when in parallel mode', function() {
+      it('reruns test when watched test file is touched', function() {
+        const testFile = path.join(tempDir, 'test.js');
+        copyFixture('__default__', testFile);
+
+        return runMochaWatch(['--parallel', testFile], tempDir, () => {
+          touchFile(testFile);
+        }).then(results => {
+          expect(results, 'to have length', 2);
+        });
+      });
+    });
+
     it('reruns test when file matching --watch-files changes', function() {
       const testFile = path.join(tempDir, 'test.js');
       copyFixture('__default__', testFile);