Make bare test modules actually reuse the outer install

millbun/integration/src/mill/bun/BunTypeScriptIntegrationTests.scala

@@ -243,6 +243,10 @@ object BunTypeScriptIntegrationTests extends TestSuite {
       val tester = this.tester("typescript-tests")
       assert(tester.eval("app.test.npmInstall").isSuccess)
       assert(!os.exists(tester.workspacePath / "test" / "bun.lock"))
+      // And must actually reuse the outer install — not run a second one that merely succeeds
+      // because the lockfile requirement happens to be off in this suite.
+      val installPath = outputPath(tester, "app.test.npmInstall")
+      assert(installPath == tester.workspacePath / "out" / "app" / "npmInstall.dest")
     }
 
     test("bunEnv") {

millbun/src/mill/javascriptlib/bun/BunTypeScriptModule.scala

@@ -409,6 +409,18 @@ trait BunTypeScriptModule extends TypeScriptModule with BunToolchainModule with
    */
   trait BunTypeScriptTests extends TypeScriptTests {
 
+    /**
+     * The outer module's Bun-specific TS toolchain, not upstream Mill's ts-node defaults.
+     *
+     * This trait extends upstream `TypeScriptTests`, so an unqualified `tsDeps()` resolves to
+     * the Node toolchain (`ts-node`, `tsconfig-paths`, `@types/node`) that the outer trait
+     * deliberately replaced. Those names always survived the outer-name filter in
+     * [[bunTestPackageJson]], so a bare test module's package.json never matched the outer's
+     * and the install-reuse path in [[npmInstall]] was unreachable — with `bunRequireLockfile`
+     * on, every bare test module demanded its own lockfile.
+     */
+    override def tsDeps: T[Seq[String]] = Task { outer.tsDeps() }
+
     /** Test timeout in milliseconds. 0 means no timeout. */
     def bunTestTimeout: T[Int] = Task { 0 }
 

millbun/src/mill/javascriptlib/bun/BunTypeScriptWebModule.scala

@@ -74,7 +74,11 @@ trait BunTypeScriptWebModule extends BunTypeScriptModule:
 
   /** Start Bun's HTML development server with source mirroring for native HMR. */
   def dev(): Command[Unit] = Task.Command {
-    val stage = webStage().path
+    // Serve from a private copy: the sync thread mirrors live edits (but never deletions) into
+    // the serving root, and `bundle` builds from the same cached stage — mutating it in place
+    // would let a file created and deleted during a dev session ship in the production bundle.
+    val stage = Task.dest / "stage"
+    mill.bun.BunToolchainModule.copyTree(webStage().path, stage)
     val entries = BunWebSupport.htmlEntries(webEntryPoints(), moduleDir, stage)
     val syncRoots = (sources() ++ generatedSources() ++ resources() ++ webEntryPoints() ++ webPublicSources())
       .filter(ref => os.exists(ref.path) && ref.path.startsWith(moduleDir))