Don't inject the autoreload tag when headers have already been sent

operations/development-server.js

@@ -18,6 +18,9 @@ function monkeyPatchEnd(res) {
 		let prefix;
 
 		if (originalChunk == null || typeof originalChunk === "string" || Buffer.isBuffer(originalChunk)) {
+			if (!res.headersSent) {
+				// If headers have already been sent, we'll just have to hope that the browser will still look at our appended tag, as we can't change the response size anymore...
+				// TODO: Make this more robust in the future
 				let typeHeader = res.getHeader("content-type");
 
 				if (typeHeader != null && typeHeader.startsWith("text/html")) {
@@ -30,6 +33,7 @@ function monkeyPatchEnd(res) {
 		
 					prefix = reloadClientTag;
 				}
+			}
 			
 			// Reset the `end` method back to the original method; we don't need to get in the way anymore
 			res.end = end;