fix: preserve user-configured NO_PROXY when loopback already covered

Only restore env vars when we actually modified them (noProxyDidModify
flag). Prevents silently deleting a user's NO_PROXY that already
contains loopback entries. Added regression test.

src/browser/cdp-proxy-bypass.test.ts

@@ -248,3 +248,30 @@ describe("withNoProxyForLocalhost reverse exit order", () => {
     }
   });
 });
+
+describe("withNoProxyForLocalhost preserves user-configured NO_PROXY", () => {
+  it("does not delete NO_PROXY when loopback entries already present", async () => {
+    const userNoProxy = "localhost,127.0.0.1,[::1],myhost.internal";
+    process.env.NO_PROXY = userNoProxy;
+    process.env.no_proxy = userNoProxy;
+    process.env.HTTP_PROXY = "http://proxy:8080";
+
+    try {
+      const { withNoProxyForLocalhost } = await import("./cdp-proxy-bypass.js");
+
+      await withNoProxyForLocalhost(async () => {
+        // Should not modify since loopback is already covered
+        expect(process.env.NO_PROXY).toBe(userNoProxy);
+        return "ok";
+      });
+
+      // After call completes, user's NO_PROXY must still be intact
+      expect(process.env.NO_PROXY).toBe(userNoProxy);
+      expect(process.env.no_proxy).toBe(userNoProxy);
+    } finally {
+      delete process.env.HTTP_PROXY;
+      delete process.env.NO_PROXY;
+      delete process.env.no_proxy;
+    }
+  });
+});

src/browser/cdp-proxy-bypass.ts

@@ -63,6 +63,7 @@ let savedNoProxy: string | undefined;
 let savedNoProxyLower: string | undefined;
 
 const LOOPBACK_ENTRIES = "localhost,127.0.0.1,[::1]";
+let noProxyDidModify = false;
 
 function noProxyAlreadyCoversLocalhost(): boolean {
   const current = process.env.NO_PROXY || process.env.no_proxy || "";
@@ -86,13 +87,14 @@ export async function withNoProxyForLocalhost<T>(fn: () => Promise<T>): Promise<
     const extended = current ? `${current},${LOOPBACK_ENTRIES}` : LOOPBACK_ENTRIES;
     process.env.NO_PROXY = extended;
     process.env.no_proxy = extended;
+    noProxyDidModify = true;
   }
 
   try {
     return await fn();
   } finally {
     noProxyRefCount--;
-    if (noProxyRefCount === 0) {
+    if (noProxyRefCount === 0 && noProxyDidModify) {
       if (savedNoProxy !== undefined) {
         process.env.NO_PROXY = savedNoProxy;
       } else {
@@ -105,6 +107,7 @@ export async function withNoProxyForLocalhost<T>(fn: () => Promise<T>): Promise<
       }
       savedNoProxy = undefined;
       savedNoProxyLower = undefined;
+      noProxyDidModify = false;
     }
   }
 }