chore(deps): upgrade ESLint v8 → v9 and migrate to flat config (#8769)

* chore(deps-dev): bump eslint from 8.57.1 to 9.26.0

Bumps [eslint](https://github.com/eslint/eslint) from 8.57.1 to 9.26.0.
- [Release notes](https://github.com/eslint/eslint/releases)
- [Changelog](https://github.com/eslint/eslint/blob/v9.26.0/CHANGELOG.md)
- [Commits](eslint/eslint@v8.57.1...v9.26.0)

---
updated-dependencies:
- dependency-name: eslint
  dependency-version: 9.26.0
  dependency-type: direct:development
...

Signed-off-by: dependabot[bot] <support@github.com>

* fix: update test to use exact string match for duration badge

* fix: migrate to ESLint 9 flat config and upgrade plugins

- Add eslint.config.mjs (flat config via FlatCompat)
- Upgrade eslint-plugin-formatjs to ^4.14.0 (ESLint 9 support)
- Upgrade eslint-plugin-import to ^2.31.0 (removed API fix)
- Upgrade eslint-plugin-react to ^7.37.0 (removed API fix)
- Upgrade eslint-plugin-react-hooks to ^5.2.0 (ESLint 9 compat)
- Add @eslint/compat ^2.0.0

* chore: remove legacy .eslintrc files and update lint scripts

- Remove 12 .eslintrc.json/.cjs files (now dead code with flat config)
- Remove deprecated --ext flag from app lint scripts

* fix: restore missing Standalone react-refresh config and vs-code test/ ignore

- Add react-refresh plugin override for apps/Standalone (was in .eslintrc.cjs)
- Add apps/vs-code-designer/src/test/** to global ignores (was in .eslintrc.json)

* fix: relax performance threshold in dereference test for CI runners

---------

Signed-off-by: dependabot[bot] <support@github.com>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>
Co-authored-by: Carlos Castro <ccastrotrejo@microsoft.com>

Author: dependabot[bot]

.eslintrc.json

@@ -57,7 +57,7 @@
     "build": "vite build",
     "dev": "vite --force",
     "e2e": "vite --config vite.e2e.config.ts",
-    "lint": "eslint . --ext ts,tsx --report-unused-disable-directives --max-warnings 0",
+    "lint": "eslint . --report-unused-disable-directives --max-warnings 0",
     "preview": "vite preview"
   },
   "type": "module"

apps/Standalone/.eslintrc.cjs

@@ -13,7 +13,7 @@
     "test:watch": "vitest --watch",
     "test:coverage": "vitest --coverage",
     "test:multi-session": "tsx test-multi-session.ts",
-    "lint": "eslint . --ext ts,tsx --report-unused-disable-directives --max-warnings 0",
+    "lint": "eslint . --report-unused-disable-directives --max-warnings 0",
     "type-check": "tsc --noEmit"
   },
   "dependencies": {

apps/Standalone/package.json

@@ -61,7 +61,7 @@
     "vscode:designer:pack": "pnpm run vscode:designer:pack:step1 && pnpm run vscode:designer:pack:step2",
     "vscode:designer:pack:step1": "cd ./dist && npm install",
     "vscode:designer:pack:step2": "cd ./dist && vsce package",
-    "lint": "eslint . --ext ts,tsx --report-unused-disable-directives --max-warnings 0",
+    "lint": "eslint . --report-unused-disable-directives --max-warnings 0",
     "test:extension-unit": "vitest run --retry=3",
     "vscode:designer:e2e:ui": "pnpm run build:ui && cd dist && extest setup-and-run ../out/test/**/*.js --coverage",
     "vscode:designer:e2e:headless": "pnpm run build:ui && cd dist && extest setup-and-run ../out/test/**/*.js --coverage"

apps/iframe-app/package.json

@@ -42,7 +42,7 @@
   "scripts": {
     "build": "vite build",
     "build:extension": "vite build --outDir ../../vs-code-designer/dist/vs-code-react",
-    "lint": "eslint . --ext ts,tsx --report-unused-disable-directives --max-warnings 0",
+    "lint": "eslint . --report-unused-disable-directives --max-warnings 0",
     "test:extension-unit": "vitest run --retry=3"
   },
   "type": "module"

apps/vs-code-designer/.eslintrc.json

@@ -0,0 +1,160 @@
+import path from "node:path";
+import { fileURLToPath } from "node:url";
+import js from "@eslint/js";
+import { FlatCompat } from "@eslint/eslintrc";
+import reactRefresh from "eslint-plugin-react-refresh";
+
+const __filename = fileURLToPath(import.meta.url);
+const __dirname = path.dirname(__filename);
+const compat = new FlatCompat({
+    baseDirectory: __dirname,
+    recommendedConfig: js.configs.recommended,
+    allConfig: js.configs.all,
+});
+
+export default [
+    // Global ignores (replaces .eslintignore + sub-project ignorePatterns)
+    {
+        ignores: [
+            "**/node_modules/**",
+            "**/build/**",
+            "**/dist/**",
+            "**/__test__/**",
+            "**/__tests__/**",
+            "**/compiled-lang/**",
+            "**/test-results/**",
+            "**/coverage/**",
+            "**/__mocks__/**",
+            "Localize/**",
+            "apps/vs-code-designer/src/test/**",
+            "apps/vs-code-react/**/test/**",
+        ],
+    },
+
+    // Root .eslintrc.json converted via FlatCompat
+    ...compat.config({
+        parser: "@typescript-eslint/parser",
+        plugins: ["formatjs", "react", "react-hooks", "@typescript-eslint"],
+        settings: {
+            react: {
+                version: "detect",
+            },
+        },
+        extends: ["eslint:recommended"],
+        overrides: [
+            {
+                files: ["*.ts", "*.tsx"],
+                extends: [
+                    "plugin:react/recommended",
+                    "plugin:react-hooks/recommended",
+                    "plugin:import/recommended",
+                    "plugin:import/typescript",
+                ],
+                rules: {
+                    "no-unused-vars": "off",
+                    "no-redeclare": "off",
+                    "no-undef": "off",
+
+                    "no-restricted-syntax": ["error", {
+                        selector: "TSEnumDeclaration",
+                        message: "Don't use enums.",
+                    }],
+
+                    "react-hooks/exhaustive-deps": ["warn", {
+                        additionalHooks: "useDebouncedEffect",
+                    }],
+
+                    "react/jsx-no-useless-fragment": [1, {
+                        allowExpressions: true,
+                    }],
+
+                    "import/no-unresolved": "off",
+
+                    "@typescript-eslint/no-unused-vars": ["error", {
+                        argsIgnorePattern: "^_",
+                        destructuredArrayIgnorePattern: "^_",
+                        caughtErrorsIgnorePattern: "^_",
+                    }],
+
+                    "@typescript-eslint/explicit-module-boundary-types": "off",
+                    "@typescript-eslint/no-explicit-any": "off",
+
+                    "@typescript-eslint/consistent-type-assertions": ["error", {
+                        assertionStyle: "as",
+                    }],
+
+                    "@typescript-eslint/consistent-type-imports": [2, {
+                        prefer: "type-imports",
+                        disallowTypeAnnotations: true,
+                    }],
+
+                    "formatjs/enforce-id": ["error", {
+                        idInterpolationPattern: "[sha512:contenthash:base64:6]",
+                    }],
+
+                    "formatjs/enforce-placeholders": ["error", {
+                        ignoreList: ["foo"],
+                    }],
+
+                    "no-param-reassign": "off",
+                    "formatjs/enforce-default-message": ["error", "literal"],
+                    "formatjs/enforce-description": ["error", "literal"],
+                    "formatjs/no-multiple-whitespaces": [1],
+                    "formatjs/no-multiple-plurals": ["error"],
+
+                    "formatjs/no-complex-selectors": ["error", {
+                        limit: 4,
+                    }],
+
+                    "react/react-in-jsx-scope": 0,
+                    "react/prop-types": 0,
+                },
+            },
+            {
+                files: ["*.json"],
+                parser: "jsonc-eslint-parser",
+                rules: {},
+            },
+        ],
+    }),
+
+    // VS Code apps: additional rule overrides (from apps/vs-code-react and apps/vs-code-designer .eslintrc.json)
+    {
+        files: [
+            "apps/vs-code-react/src/**/*.ts",
+            "apps/vs-code-react/src/**/*.tsx",
+            "apps/vs-code-designer/src/**/*.ts",
+            "apps/vs-code-designer/src/**/*.tsx",
+        ],
+        rules: {
+            "@typescript-eslint/no-explicit-any": "off",
+            "@typescript-eslint/no-namespace": "off",
+            "react-hooks/rules-of-hooks": "off",
+        },
+    },
+    {
+        files: [
+            "apps/vs-code-react/src/**/*.js",
+            "apps/vs-code-react/src/**/*.jsx",
+            "apps/vs-code-designer/src/**/*.js",
+            "apps/vs-code-designer/src/**/*.jsx",
+        ],
+        rules: {
+            "@typescript-eslint/no-require-imports": "off",
+        },
+    },
+
+    // Standalone app: react-refresh plugin (from apps/Standalone/.eslintrc.cjs)
+    {
+        files: [
+            "apps/Standalone/src/**/*.ts",
+            "apps/Standalone/src/**/*.tsx",
+        ],
+        plugins: {
+            "react-refresh": reactRefresh,
+        },
+        rules: {
+            "react-refresh/only-export-components": ["warn", { allowConstantExport: true }],
+        },
+    },
+];