diff --git a/Dockerfile b/Dockerfile
index 4b81875..2a53990 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -18,9 +18,8 @@ COPY . .
 RUN yarn run lint
 RUN script/check-censored
 
-# Test phase, same shape and for the same reason: the jest suite (its worker
-# cap is in package.json), rerun verbose on failure, then
-# script/test-verify-build.
+# Test phase, same shape and for the same reason: the jest suite, rerun
+# verbose on failure, then script/test-verify-build.
 #
 # node:22-slim (22.x LTS), 2026-02-24
 FROM node@sha256:5373f1906319b3a1f291da5d102f4ce5c77ccbe29eb637f072b6c7b70443fc36 AS test
@@ -33,9 +32,9 @@ RUN script/bootstrap
 
 COPY . .
 
-RUN timeout 90 yarn run test || \
+RUN yarn run test || \
     { echo "--- Rerunning with --verbose for details ---"; \
-      timeout 90 yarn run test:verbose; exit 1; }
+      yarn run test:verbose; exit 1; }
 RUN script/test-verify-build
 
 # Development environment with the extension built, and the last stage: a
diff --git a/README.md b/README.md
index 9f38fe9..32146ca 100644
--- a/README.md
+++ b/README.md
@@ -1761,8 +1761,8 @@ The `Makefile` targets are thin shims over `script/`:
   `package.json`.
 - `script/setup`: `script/bootstrap`, then the git pre-commit hook.
 - `script/test`: build the `test` stage of the `Dockerfile` with `--no-cache`.
-  The stage runs the jest suite, stopped at 90 seconds and rerun verbose on
-  failure, then `script/test-verify-build`.
+  The stage runs the jest suite, rerun verbose on failure, then
+  `script/test-verify-build`.
 - `script/lint`: build the `lint` stage with `--no-cache`. The stage runs
   ESLint, the prettier check and `script/check-censored`. Linting runs only in
   Docker.
@@ -1861,9 +1861,10 @@ have left a complete, loadable debug bundle behind. `make build-debug` keeps its
 
 ### Tests
 
-`make test` runs the jest suite in Docker. The suite must finish within 60
-seconds; the test stage stops it at 90. `make check` runs the tests, the lint
-stage and the formatting check.
+`make test` runs the jest suite in Docker, in one process. The suite must finish
+within 60 seconds. A test that leaves a timer or another open handle running
+fails the run. `make check` runs the tests, the lint stage and the formatting
+check.
 
 Some unit tests pin facts stated elsewhere in this file:
 
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..f9f62f4 100644
--- a/package.json
+++ b/package.json
@@ -6,8 +6,8 @@
     "license": "GPL-3.0",
     "private": true,
     "scripts": {
-        "test": "node script/lib/jest.js --maxWorkers=3",
-        "test:verbose": "node script/lib/jest.js --maxWorkers=3 --verbose",
+        "test": "node script/lib/jest.js --runInBand",
+        "test:verbose": "node script/lib/jest.js --runInBand --verbose",
         "build": "node build.js",
         "lint": "eslint . && prettier --check .",
         "fmt": "prettier --write .",
diff --git a/script/lib/jest.js b/script/lib/jest.js
index 96ba5dc..5e4d545 100644
--- a/script/lib/jest.js
+++ b/script/lib/jest.js
@@ -1,8 +1,8 @@
-// Runs jest with the arguments it is given, and fails the run when jest had to
-// force-stop the worker process that ran a test file. Jest itself only prints a
-// warning when that happens, and its run still passes. The warning means a test
-// left a timer or another open handle running. The test scripts in package.json
-// run jest through this file.
+// Runs jest with the arguments it is given, and fails the run at once when a
+// test left a timer or another open handle running. Such a handle keeps this
+// process alive after the last test, so the run would hang. Jest itself only
+// prints a warning one second after the run, and then keeps waiting. The test
+// scripts in package.json run jest through this file, in this one process.
 //
 // Jest runs inside this process and writes to the terminal directly, so its
 // output looks the same as a plain jest run. This file only checks each write
@@ -10,31 +10,23 @@
 
 "use strict";
 
-// The start of the warning that jest prints on stderr after it force-stops a
-// worker process.
-const WORKER_WARNING = "A worker process has failed to exit gracefully";
-
-let forceStopped = false;
+// The start of the warning that jest prints on stderr when the process has not
+// exited one second after the test run.
+const OPEN_HANDLE_WARNING =
+    "Jest did not exit one second after the test run has completed.";
 
 const writeStderr = process.stderr.write.bind(process.stderr);
 process.stderr.write = (chunk, ...rest) => {
-    if (String(chunk).includes(WORKER_WARNING)) {
-        forceStopped = true;
-    }
-    return writeStderr(chunk, ...rest);
-};
-
-// Runs after jest has finished, as the process exits.
-process.on("exit", () => {
-    if (forceStopped) {
-        console.error(
-            "script/lib/jest.js: failing the run, because jest force-stopped " +
-                "a worker process. A test left a timer or another open " +
-                "handle running.",
+    const written = writeStderr(chunk, ...rest);
+    if (String(chunk).includes(OPEN_HANDLE_WARNING)) {
+        writeStderr(
+            "script/lib/jest.js: failing the run, because a test left a " +
+                "timer or another open handle running.\n",
         );
-        process.exitCode = 1;
+        process.exit(1);
     }
-});
+    return written;
+};
 
 // Jest's own command-line program. It reads its arguments from process.argv.
 require("jest/bin/jest");
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..dbe79ee
--- /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 a test left an open handle, instead of hanging on it.
+
+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 run ends about one second after its last test. Without the check in
+    // script/lib/jest.js it would never end, and the timeout stops it.
+    const result = spawnSync(
+        process.execPath,
+        [
+            "script/lib/jest.js",
+            "--runInBand",
+            "--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(
+        "a test left a timer or another open handle running",
+    );
+});