fix(har): off-by-one in WebSocket frame header size calculation

packages/playwright-core/src/server/har/harTracer.ts

@@ -489,7 +489,7 @@ export class HarTracer {
           // - there are always 4 bytes for the masking key (see <https://www.rfc-editor.org/info/rfc6455/#section-5.1>)
           // - there may be an additional 16 or 64 bits for payload length if it's too long to fit in the above 7 bits (or if it also can't fit in 16 bits)
           let headerSize = 6;
-          if (length > 2 ** 16)
+          if (length >= 2 ** 16)
             headerSize += 8;
           else if (length > 125)
             headerSize += 2;

tests/library/har-websocket.spec.ts

@@ -232,6 +232,34 @@ it('should include gigantic websocket messages', async ({ contextFactory, server
   expect(messages[0].time).toBeLessThanOrEqual(messages[1].time);
 });
 
+it('should use 64-bit extended length for exactly 2**16 byte websocket payload', async ({ contextFactory, server }, testInfo) => {
+  // RFC 6455 Section 5.2: 16-bit extended length covers 126-65535.
+  // Payloads of exactly 65536 (2**16) bytes require the 64-bit extended length field.
+  const incoming = 'x'.repeat(2 ** 16);
+  const outgoing = 'outgoing';
+
+  server.onceWebSocketConnection(ws => {
+    ws.on('message', () => ws.send(incoming));
+  });
+
+  const { page, getLog } = await pageWithHar(contextFactory, testInfo);
+  await page.goto(server.EMPTY_PAGE);
+
+  const wsUrl = `ws://${server.HOST}/ws`;
+  const closed = page.evaluate(({ url, outgoing }) => new Promise<void>(resolve => {
+    const ws = new WebSocket(url);
+    ws.addEventListener('open', () => ws.send(outgoing));
+    ws.addEventListener('message', () => ws.close());
+    ws.addEventListener('close', () => resolve());
+  }), { url: wsUrl, outgoing });
+  await closed;
+  const log = await getLog();
+
+  const wsEntry = log.entries.find(e => e.request.url === wsUrl)! as Entry;
+  // 6 (base header) + 8 (64-bit extended length) for payload of exactly 2**16 bytes.
+  expect(wsEntry.response._transferSize).toBe(responseHeadersSize(wsEntry.response.headers) + 6 + 8 + incoming.length);
+});
+
 it('should include binary websocket messages', async ({ contextFactory, server }, testInfo) => {
   const incoming = [0x01, 0x02, 0x03, 0x04];
   const outgoing = [0x05, 0x06, 0x07, 0x08];