fix(typescript): gate temp outDir cleanup to non-watch; cleanup in closeWatcher; harden outDir exclusion\n\n- Clean temp outDir only when not watching; perform final cleanup in closeWatcher to avoid watch-mode churn\n- Robust containment check for excluding outDir using drive-root equality + path.relative + !isAbsolute (handles Windows cross-drive)\n- When filterRoot=false, use CWD fallback for containment to avoid over-excluding sources (e.g., outDir='.')\n- Docs: clarify cleanup timing in README (non-watch vs watch)

Author: CharlieHelps

packages/typescript/README.md

@@ -79,7 +79,7 @@ Default: `null`
 
 A [picomatch pattern](https://github.com/micromatch/picomatch), or array of patterns, which specifies the files in the build the plugin should operate on. By default all `.ts` and `.tsx` files are targeted. If your `tsconfig.json` (or plugin `compilerOptions`) sets `allowJs: true`, the default include expands to also cover `.js`, `.jsx`, `.mjs`, and `.cjs` files so that JavaScript sources are downleveled by TypeScript as well.
 
-> Note: When `allowJs` is enabled and no `outDir` is configured, this plugin creates a temporary output directory for TypeScript emit to avoid TS5055 (overwriting input files). The directory is excluded from processing and removed after the build.
+> Note: When `allowJs` is enabled and no `outDir` is configured, this plugin creates a temporary output directory for TypeScript emit to avoid TS5055 (overwriting input files). The directory is excluded from processing and removed after the build in non‑watch runs, and when the watcher stops in watch mode.
 
 ### `filterRoot`
 

packages/typescript/src/index.ts

@@ -69,18 +69,26 @@ export default function typescript(options: RollupTypescriptOptions = {}): Plugi
   const effectiveOutDir = parsedOptions.options.outDir
     ? path.resolve(parsedOptions.options.outDir)
     : null;
-  const filterBase = (filterRoot ?? parsedOptions.options.rootDir) || null;
+  // Determine the base directory used for containment checks. When
+  // `filterRoot === false`, Rollup's pattern resolver does not resolve
+  // include/exclude patterns against a directory. In that edge case,
+  // use CWD as a sensible fallback to avoid over-excluding sources
+  // (e.g., when outDir='.'), while still preventing feedback loops.
+  const willResolvePatterns = filterRoot !== false;
+  const filterBase = willResolvePatterns ? filterRoot ?? parsedOptions.options.rootDir : null;
   const filterBaseAbs = filterBase ? path.resolve(filterBase) : null;
   if (effectiveOutDir) {
-    // Avoid excluding sources: skip when the filter base (rootDir) lives inside outDir
-    // Use path.relative for a robust cross-platform containment check.
-    const outDirContainsFilterBase = filterBaseAbs
-      ? (() => {
-          const rel = path.relative(effectiveOutDir, filterBaseAbs);
-          // rel === '' -> same dir; rel starts with '..' -> outside
-          return rel === '' || !(rel === '..' || rel.startsWith(`..${path.sep}`));
-        })()
-      : false;
+    // Avoid excluding sources: skip when the filter base (rootDir) lives inside outDir.
+    // Use a robust cross-platform containment check that handles Windows
+    // different-drive cases where path.relative may return an absolute path.
+    const baseForContainment = filterBaseAbs ?? process.cwd();
+    const outDirContainsFilterBase = (() => {
+      // Different roots (e.g., Windows drive letters) can never be parent/child
+      if (path.parse(effectiveOutDir).root !== path.parse(baseForContainment).root) return false;
+      const rel = path.relative(effectiveOutDir, baseForContainment);
+      // rel === '' => same dir; if absolute or starts with '..', it's outside
+      return rel === '' || (!rel.startsWith('..') && !path.isAbsolute(rel));
+    })();
     if (!outDirContainsFilterBase) {
       filterExclude.push(normalizePath(path.join(effectiveOutDir, '**')));
     }
@@ -146,7 +154,20 @@ export default function typescript(options: RollupTypescriptOptions = {}): Plugi
         // ESLint doesn't understand optional chaining
         // eslint-disable-next-line
         program?.close();
+        if (autoOutDir) {
+          try {
+            fs.rmSync(autoOutDir, { recursive: true, force: true });
+          } catch {
+            // ignore cleanup failures
+          }
+          autoOutDir = null;
+        }
       }
+    },
+
+    // Ensure we clean up any auto-created outDir exactly once when a watch
+    // session ends, avoiding churn during incremental rebuilds.
+    closeWatcher() {
       if (autoOutDir) {
         try {
           fs.rmSync(autoOutDir, { recursive: true, force: true });

packages/typescript/test/fixtures/incremental-watch-off/main.ts

@@ -4,3 +4,5 @@ const answer: AnswerToQuestion = '42';
 
 // eslint-disable-next-line no-console
 console.log(`the answer is ${answer}`);
+
+export const REBUILD_WITH_WATCH_OFF = 1;