From 60e15cb6e16637e1f05c349a54692b243f745cdb Mon Sep 17 00:00:00 2001
From: Raymond Hill <gorhill@users.noreply.github.com>
Date: Fri, 24 Oct 2025 12:26:37 -0400
Subject: [PATCH] Improve `prevent-fetch` scriptlet

Related issue:
https://github.com/uBlockOrigin/uBlock-issues/issues/3828

Ability to set header values for the trusted version of the
scriptlet. Example:

..##+js(trusted-prevent-fetch, target, length:100, '{"headers":{"content-type": "image/png"}}')
---
 src/js/resources/prevent-fetch.js | 15 ++++++++++-----
 1 file changed, 10 insertions(+), 5 deletions(-)

diff --git a/src/js/resources/prevent-fetch.js b/src/js/resources/prevent-fetch.js
index cfd66c0ac..ba2565a69 100644
--- a/src/js/resources/prevent-fetch.js
+++ b/src/js/resources/prevent-fetch.js
@@ -65,9 +65,14 @@ function preventFetchFn(
     const responseProps = {
         statusText: { value: 'OK' },
     };
+    const responseHeaders = {};
     if ( /^\{.*\}$/.test(responseType) ) {
         try {
             Object.entries(JSON.parse(responseType)).forEach(([ p, v ]) => {
+                if ( p === 'headers' && trusted ) {
+                    Object.assign(responseHeaders, v);
+                    return;
+                }
                 if ( validResponseProps[p] === undefined ) { return; }
                 if ( validResponseProps[p].includes(v) === false ) { return; }
                 responseProps[p] = { value: v };
@@ -120,11 +125,11 @@ function preventFetchFn(
         }
         return Promise.resolve(generateContentFn(trusted, responseBody)).then(text => {
             safe.uboLog(logPrefix, `Prevented with response "${text}"`);
-            const response = new Response(text, {
-                headers: {
-                    'Content-Length': text.length,
-                }
-            });
+            const headers = Object.assign({}, responseHeaders);
+            if ( headers['content-length'] === undefined ) {
+                headers['content-length'] = text.length;
+            }
+            const response = new Response(text, { headers });
             const props = Object.assign(
                 { url: { value: details.url } },
                 responseProps