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

operations/development-server.js

@@ -18,17 +18,21 @@ function monkeyPatchEnd(res) {
 		let prefix;
 
 		if (originalChunk == null || typeof originalChunk === "string" || Buffer.isBuffer(originalChunk)) {
-			let typeHeader = res.getHeader("content-type");
+			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")) {
-				let contentLength = res.getHeader("content-length");
-				
-				if (contentLength != null) {
-					// Compensate for the additional bytes introduced by our injected script tag
-					res.setHeader("content-length", parseInt(contentLength) + reloadClientTag.length);
+				if (typeHeader != null && typeHeader.startsWith("text/html")) {
+					let contentLength = res.getHeader("content-length");
+					
+					if (contentLength != null) {
+						// Compensate for the additional bytes introduced by our injected script tag
+						res.setHeader("content-length", parseInt(contentLength) + reloadClientTag.length);
+					}
+		
+					prefix = reloadClientTag;
 				}
-	
-				prefix = reloadClientTag;
 			}
 			
 			// Reset the `end` method back to the original method; we don't need to get in the way anymore