Defer HTTP completion callbacks to next tick

Client.makeRequest used to call self.perform(0) after handing the transfer
to libcurl. That perform() does two things: drives curl_multi_perform (so
bytes hit the wire) AND drains curl_multi_info_read messages, which is
what fires the user-facing header/data/done callbacks.

The issue is that, even in non-cache cases, a request could be immediately
resolved in libcurl, and thus callbacks executed synchronously.

By only calling `curl_multi_perform` on a new request, we prevent this from
happening.

src/browser/HttpClient.zig

@@ -611,7 +611,15 @@ fn makeRequest(self: *Client, conn: *http.Connection, transfer: *Transfer) anyer
             return err;
         };
     }
-    _ = try self.perform(0);
+
+    // Start the request (and move along any other request). This used to call
+    // self.perform(0) but that can also execute callbacks. Normally, that
+    // wouldn't be so bad. But curl can synchronously fire callbacks for the
+    // request we JUST added, which we do not want (it results in incorrect
+    // execution).
+    self.performing = true;
+    defer self.performing = false;
+    _ = try self.handles.perform();
 }
 
 pub const PerformStatus = enum {

src/browser/tests/worker/worker.html

@@ -337,3 +337,26 @@
     });
   }
 </script>
+
+<script id="worker_post_before_load" type=module>
+  // Per spec, messages posted before the worker has finished loading must
+  // be buffered and delivered once onmessage is registered. Without the
+  // pending-message queue this hangs: postMessage fires immediately after
+  // new Worker, while the worker script's HTTP fetch is still in flight,
+  // so by the time the worker dispatches the message there is no
+  // onmessage listener yet and the message gets silently dropped.
+  {
+    const state = await testing.async();
+    const worker = new Worker('./echo-worker.js');
+    worker.onmessage = function(event) {
+      state.resolve(event.data);
+    };
+    // No setTimeout — post right now while the script is still loading.
+    worker.postMessage({ greeting: 'before-load' });
+
+    await state.done((response) => {
+      testing.expectEqual('before-load', response.echo.greeting);
+      testing.expectEqual('worker', response.from);
+    });
+  }
+</script>