diff --git a/eslint.config.js b/eslint.config.js
index 1c420d5..64122df 100644
--- a/eslint.config.js
+++ b/eslint.config.js
@@ -135,9 +135,14 @@ module.exports = [
         },
     },
 
-    // Unit tests, and the helpers they require: jest on node.
+    // Unit tests, the helpers they require, and the test files they hand to
+    // jest: jest on node.
     {
-        files: ["tests/**/*.test.js", "tests/support/**/*.js"],
+        files: [
+            "tests/**/*.test.js",
+            "tests/support/**/*.js",
+            "tests/fixtures/**/*.js",
+        ],
         languageOptions: {
             ...commonjs,
             globals: { ...globals.node, ...globals.jest },
diff --git a/package.json b/package.json
index 2080619..403a6f0 100644
--- a/package.json
+++ b/package.json
@@ -16,7 +16,8 @@
     "jest": {
         "setupFiles": [
             "<rootDir>/tests/support/navigatorLocks.js"
-        ]
+        ],
+        "workerIdleMemoryLimit": "200MB"
     },
     "devDependencies": {
         "@eslint/js": "10.0.1",
diff --git a/tests/fixtures/leavesIntervalRunning.js b/tests/fixtures/leavesIntervalRunning.js
new file mode 100644
index 0000000..a0229cb
--- /dev/null
+++ b/tests/fixtures/leavesIntervalRunning.js
@@ -0,0 +1,7 @@
+// A test file that starts an interval and never clears it.
+// tests/jestRunner.test.js runs script/lib/jest.js on this file alone. The
+// name does not end in .test.js, so the normal run never picks it up.
+
+test("starts an interval and never clears it", () => {
+    setInterval(() => {}, 1000);
+});
diff --git a/tests/jestRunner.test.js b/tests/jestRunner.test.js
new file mode 100644
index 0000000..38299d0
--- /dev/null
+++ b/tests/jestRunner.test.js
@@ -0,0 +1,29 @@
+// script/lib/jest.js, which package.json's test scripts run, fails a run in
+// which jest force-stopped a worker process because a test left an open
+// handle.
+
+const { spawnSync } = require("child_process");
+const path = require("path");
+
+const root = path.join(__dirname, "..");
+
+test("a test that leaves an interval running fails the run", () => {
+    // The memory limit in package.json makes jest run even this one file in a
+    // worker process. The interval keeps that worker from exiting, so jest
+    // force-stops it after the run.
+    const result = spawnSync(
+        process.execPath,
+        [
+            "script/lib/jest.js",
+            "--maxWorkers=1",
+            "--testMatch",
+            "<rootDir>/tests/fixtures/leavesIntervalRunning.js",
+        ],
+        { cwd: root, encoding: "utf8", timeout: 20000 },
+    );
+
+    expect(result.signal).toBeNull();
+    expect(result.status).toBe(1);
+    expect(result.stderr).toContain("1 passed");
+    expect(result.stderr).toContain("jest force-stopped a worker process");
+});