Vaadin Flow silent WebSocket to Long Polling fallback

When Vaadin Flow is configured with pushMode=AUTO, the framework attempts a WebSocket connection before falling back to long polling if the handshake fails. In environments behind strict proxies or utilizing self-signed certificates, the WebSocket handshake may time out silently without triggering an explicit client-side error in the browser console.

While the vaadin.push.debug=true property provides some protocol visibility, the internal logic determining the timing and reason for the fallback is not fully exposed through public APIs. This makes it difficult to distinguish between a network-level block, a TLS-specific handshake failure, or a browser-side timeout limit.

What specific server-side logs indicate the reason for a fallback when a WebSocket handshake times out, and is there a configuration property to adjust the timeout duration for the WebSocket handshake before the client switches to long polling?