chore: Upgrade to Vitest 4 and regroup tests for performance (#2618)

vite.config.ts

@@ -23,23 +23,6 @@ const EXCLUSIONS = [
 ];
 
 const TEST_DIRECTORY = 'tests';
-const INCLUSIONS = [`${TEST_DIRECTORY}/**/*.test.ts`];
-
-// For test performance reasons, tests are split into two groups:
-//
-// - `shared`: run non-isolated, so they share one loaded copy of the source
-//   tree instead of each re-importing it.
-// - `isolated`: given their own module registry per file, because sharing one
-//   would change their behaviour. A file calling `vi.mock()` cannot replace a
-//   module an earlier file already loaded unmocked; a file loading the template
-//   renderer reads browser globals as the renderer loads; and a file needing a
-//   DOM leaves modules holding a `window` that is torn down when it finishes,
-//   which breaks any later file that reaches one of those modules.
-//
-// Absent this variable every test file runs isolated, which is both the safe
-// default and what coverage requires (istanbul only counts a module's top-level
-// code the first time it runs).
-const GROUP = process.env.VITEST_GROUP;
 
 const findTestFiles = (directory: string): string[] => {
   const files: string[] = [];
@@ -54,16 +37,21 @@ const findTestFiles = (directory: string): string[] => {
   return files;
 };
 
-const REQUIRE_ISOLATION_REGEXP =
-  /\bvi\.(do)?mock\(|loadRenderer|stubConnectedHomeAssistant|@vitest-environment\s+jsdom/;
+// Each test file is read and sorted into one of the projects below, which
+// explain what these two properties cost.
+const REQUIRE_ISOLATION_REGEXP = /\bvi\.(do)?mock\(/;
+const REQUIRE_DOM_REGEXP = /@vitest-environment\s+jsdom/;
 
-const getGroup = (file: string): string =>
-  REQUIRE_ISOLATION_REGEXP.test(readFileSync(file, 'utf-8')) ? 'isolated' : 'shared';
+const getGroup = (file: string): string => {
+  const contents = readFileSync(file, 'utf-8');
+  if (REQUIRE_ISOLATION_REGEXP.test(contents)) {
+    return 'isolated';
+  }
+  return REQUIRE_DOM_REGEXP.test(contents) ? 'shared-jsdom' : 'shared-node';
+};
 
-const getInclusions = (): string[] =>
-  GROUP
-    ? findTestFiles(TEST_DIRECTORY).filter((file) => getGroup(file) === GROUP)
-    : INCLUSIONS;
+const getInclusions = (group: string): string[] =>
+  findTestFiles(TEST_DIRECTORY).filter((file) => getGroup(file) === group);
 
 export default defineConfig({
   plugins: [svgPath()],
@@ -75,13 +63,62 @@ export default defineConfig({
         inline: ['ha-nunjucks', 'ts-py-datetime'],
       },
     },
-    include: getInclusions(),
 
     // Forked child processes start and tear down faster here than worker
     // threads, which matters when every test file needs a fresh one.
     pool: 'forks',
 
-    isolate: !GROUP || GROUP === 'isolated',
+    // Importing the source tree costs far more than running the tests in it, so
+    // files are grouped by whether they can share one loaded copy of it. Each
+    // project below is a group that can, or the one that cannot.
+    projects: [
+      {
+        extends: true,
+        test: {
+          // Nothing stops these sharing, so they run against a single loaded
+          // copy of the source tree. Most of the suite is here, and anything
+          // moved out of here pays to import that tree again.
+          name: 'shared-node',
+          include: getInclusions('shared-node'),
+          isolate: false,
+        },
+      },
+      {
+        extends: true,
+        test: {
+          // These share too, but only with each other. A module loaded under
+          // jsdom holds a reference to that DOM, so a worker that moves from a
+          // DOM file to a non-DOM one drops its loaded copy of the source tree
+          // and imports it again. A project's files are spread across workers
+          // without regard to environment, so putting both kinds in one project
+          // leaves every worker alternating between them and reloading as it
+          // goes, and the more workers there are the more often that happens. A
+          // project per environment hands each worker a run of files that all
+          // want the same one. The file-level `@vitest-environment jsdom`
+          // comments are what sort files into here, and are redundant once they
+          // arrive.
+          //
+          // They share one `document` as well, so a file that redefines part of
+          // it must leave the property configurable for the files that follow.
+          name: 'shared-jsdom',
+          include: getInclusions('shared-jsdom'),
+          environment: 'jsdom',
+          isolate: false,
+        },
+      },
+      {
+        extends: true,
+        test: {
+          // `vi.mock()` cannot replace a module that an earlier file already
+          // loaded unmocked, so these cannot share a loaded source tree with
+          // anything, including each other. They get a registry per file and
+          // pay the full import cost.
+          name: 'isolated',
+          include: getInclusions('isolated'),
+          isolate: true,
+        },
+      },
+    ],
 
     // Hide console writing to keep output clean, usual sources of noise:
     // - Unnecessary Lit dev-mode warnings.