fix(http2): send connection-level WINDOW_UPDATE in setLocalWindowSize

`setLocalWindowSize()` updated the internal connection window size but
never sent a WINDOW_UPDATE frame for stream 0 (the connection level) to
the peer. Per RFC 9113 Section 6.9, the INITIAL_WINDOW_SIZE setting only
applies to stream-level windows; the connection-level window must be
updated explicitly via WINDOW_UPDATE. This caused HTTP/2 streams to
stall after receiving 65,535 bytes (the default connection window).

Closes #26915

Co-Authored-By: Claude <noreply@anthropic.com>

src/bun.js/api/bun/h2_frame_parser.zig

@@ -2864,10 +2864,19 @@ pub const H2FrameParser = struct {
         if (this.usedWindowSize > windowSizeValue) {
             return globalObject.throwInvalidArguments("Expected windowSize to be greater than usedWindowSize", .{});
         }
+        const oldWindowSize = this.windowSize;
         this.windowSize = windowSizeValue;
         if (this.localSettings.initialWindowSize < windowSizeValue) {
             this.localSettings.initialWindowSize = windowSizeValue;
         }
+        // Send a connection-level WINDOW_UPDATE frame to the peer so it knows
+        // about the increased window.  Per RFC 9113 Section 6.9, the
+        // INITIAL_WINDOW_SIZE setting only applies to stream-level windows;
+        // the connection-level window must be updated explicitly.
+        if (windowSizeValue > oldWindowSize) {
+            const increment: u31 = @truncate(windowSizeValue - oldWindowSize);
+            this.sendWindowUpdate(0, UInt31WithReserved.init(increment, false));
+        }
         var it = this.streams.valueIterator();
         while (it.next()) |stream| {
             if (stream.usedWindowSize > windowSizeValue) {

test/regression/issue/26915.test.ts

@@ -0,0 +1,58 @@
+import { expect, test } from "bun:test";
+import http2 from "node:http2";
+
+// Regression test for https://github.com/oven-sh/bun/issues/26915
+// setLocalWindowSize() must send a connection-level WINDOW_UPDATE frame.
+// Without this, the peer's connection-level window stays at the default
+// 65,535 bytes and streams stall when receiving larger payloads.
+test("http2 client setLocalWindowSize sends connection-level WINDOW_UPDATE", async () => {
+  const payloadSize = 256 * 1024; // 256 KB - well above the 65535 default
+  const payload = Buffer.alloc(payloadSize, "x");
+
+  const { promise: serverReady, resolve: resolveServer } = Promise.withResolvers<{
+    port: number;
+    server: http2.Http2Server;
+  }>();
+
+  const server = http2.createServer();
+  server.on("stream", stream => {
+    stream.respond({ ":status": 200 });
+    stream.end(payload);
+  });
+  server.listen(0, () => {
+    const addr = server.address();
+    if (addr && typeof addr === "object") {
+      resolveServer({ port: addr.port, server });
+    }
+  });
+
+  const { port, server: srv } = await serverReady;
+
+  try {
+    const client = http2.connect(`http://localhost:${port}`, {
+      settings: { initialWindowSize: 10 * 1024 * 1024 },
+    });
+
+    client.setLocalWindowSize(10 * 1024 * 1024);
+
+    const { promise: done, resolve: resolveDone, reject: rejectDone } = Promise.withResolvers<Buffer>();
+
+    const req = client.request({ ":path": "/" });
+    const chunks: Buffer[] = [];
+    req.on("data", (chunk: Buffer) => {
+      chunks.push(chunk);
+    });
+    req.on("end", () => {
+      resolveDone(Buffer.concat(chunks));
+    });
+    req.on("error", rejectDone);
+    req.end();
+
+    const result = await done;
+    expect(result.length).toBe(payloadSize);
+
+    client.close();
+  } finally {
+    srv.close();
+  }
+});