diff --git a/.gitignore b/.gitignore
index 9c243f01..f19c6dcb 100644
--- a/.gitignore
+++ b/.gitignore
@@ -2,6 +2,7 @@
 /_build/
 /static/*.bundle.js
 /static/*.bundle.js.map
+/static/bundle.css
 
 # Build helper files.
 /.build*
diff --git a/package.json b/package.json
index d3bf3133..9e6d4c77 100644
--- a/package.json
+++ b/package.json
@@ -60,7 +60,8 @@
     "stats-webpack-plugin": "^0.7.0",
     "typescript": "3.9.3",
     "webpack": "4.41.0",
-    "webpack-cli": "3.3.2"
+    "webpack-cli": "3.3.2",
+    "why-is-node-running": "2.0.3"
   },
   "dependencies": {
     "@gristlabs/connect-sqlite3": "0.9.11",
diff --git a/test/mocha.opts b/test/mocha.opts
index ac4bcb20..59f9bdad 100644
--- a/test/mocha.opts
+++ b/test/mocha.opts
@@ -1,2 +1,3 @@
 --require source-map-support/register
+test/report-why-tests-hang
 test/init-mocha-webdriver
diff --git a/test/report-why-tests-hang.js b/test/report-why-tests-hang.js
new file mode 100644
index 00000000..716fca23
--- /dev/null
+++ b/test/report-why-tests-hang.js
@@ -0,0 +1,34 @@
+/**
+ * Mocha 4 no longer forces exit of a process after tests end, so if a setTimeout() or anything
+ * else is keeping node running, tests will hang after finishing.
+ *
+ * This helper module, always included via mocha.opts, ensures we print something if that happens.
+ * We use why-is-node-running module to print something informative.
+ */
+
+/* global after */
+
+const whyIsNodeRunning = require('why-is-node-running');
+
+function report() {
+  whyIsNodeRunning();
+  console.warn("*******************************************************");
+  console.warn("Something above prevented node from exiting on its own.");
+  console.warn("*******************************************************");
+  // We want to exit, but process.exit(1) doesn't work, since mocha catches it and insists on
+  // exiting with the test status result (which may be 0, and we need to indicate failure).
+  process.kill(process.pid, 'SIGTERM');
+}
+
+after(() => {
+  // --no-exit|-E flag is interpreted by mocha-webdriver library to start REPL on failure, and we
+  // do NOT want to output a big dump about that.
+  const noexit = process.argv.includes("--no-exit") || process.argv.includes('-E');
+  if (noexit) {
+    console.log("report-why-tests-hang silenced with --no-exit flag");
+  } else {
+    // If still hanging after 5s after tests finish, say something. Unref() ensures that THIS
+    // timeout doesn't itself keep node from exiting.
+    setTimeout(report, 5000).unref();
+  }
+});
diff --git a/yarn.lock b/yarn.lock
index d0dc66fb..4bedf649 100644
--- a/yarn.lock
+++ b/yarn.lock
@@ -5957,6 +5957,11 @@ stack-trace@0.0.x:
   resolved "https://registry.yarnpkg.com/stack-trace/-/stack-trace-0.0.10.tgz#547c70b347e8d32b4e108ea1a2a159e5fdde19c0"
   integrity sha1-VHxws0fo0ytOEI6hoqFZ5f3eGcA=
 
+stackback@0.0.2:
+  version "0.0.2"
+  resolved "https://registry.yarnpkg.com/stackback/-/stackback-0.0.2.tgz#1ac8a0d9483848d1695e418b6d031a3c3ce68e3b"
+  integrity sha1-Gsig2Ug4SNFpXkGLbQMaPDzmjjs=
+
 static-extend@^0.1.1:
   version "0.1.2"
   resolved "https://registry.yarnpkg.com/static-extend/-/static-extend-0.1.2.tgz#60809c39cbff55337226fd5e0b520f341f1fb5c6"
@@ -6894,6 +6899,13 @@ which@1.3.1, which@^1.2.14, which@^1.2.9:
   dependencies:
     isexe "^2.0.0"
 
+why-is-node-running@2.0.3:
+  version "2.0.3"
+  resolved "https://registry.yarnpkg.com/why-is-node-running/-/why-is-node-running-2.0.3.tgz#86619c2861405d3509f55268684fc85e7f4ce145"
+  integrity sha512-XmzbFN2T859avcs5qAsiiK1iu0nUpSUXRgiGsoHPcNijxhIlp1bPQWQk6ANUljDWqBtAbIR2jF1HxR0y2l2kCA==
+  dependencies:
+    stackback "0.0.2"
+
 wide-align@1.1.3, wide-align@^1.1.0:
   version "1.1.3"
   resolved "https://registry.yarnpkg.com/wide-align/-/wide-align-1.1.3.tgz#ae074e6bdc0c14a431e804e624549c633b000457"