DownloadHandler disconnects from UI on Firefox

[HerbertsVaadin]

Description

Seems like there's a connection issue on Firefox, causing delay of notifications (at some point they still appear) and temporary UI disconnect to occur (causing "Online" text to appear in the loader.
Occurs consistently after a couple of downloads.
I have not had any issues with it on Chrome.

Logged error:

2025-06-02T13:10:50.800+03:00 ERROR 53611 --- [phere-Shared-43] c.v.f.s.c.PushAtmosphereHandler          : Exception in push connection

java.io.IOException: Connection remotely closed for 7d9d4774-99c6-4995-bd0e-53d1e30d8a14
	at org.atmosphere.websocket.WebSocket.write(WebSocket.java:237) ~[atmosphere-runtime-3.0.5.slf4jvaadin1.jar:3.0.5.slf4jvaadin1]
	at org.atmosphere.websocket.WebSocket.write(WebSocket.java:227) ~[atmosphere-runtime-3.0.5.slf4jvaadin1.jar:3.0.5.slf4jvaadin1]
	at org.atmosphere.websocket.WebSocket.write(WebSocket.java:48) ~[atmosphere-runtime-3.0.5.slf4jvaadin1.jar:3.0.5.slf4jvaadin1]
	at org.atmosphere.cpr.AtmosphereResponseImpl$Stream.write(AtmosphereResponseImpl.java:955) ~[atmosphere-runtime-3.0.5.slf4jvaadin1.jar:3.0.5.slf4jvaadin1]
	at org.atmosphere.handler.AbstractReflectorAtmosphereHandler.onStateChange(AbstractReflectorAtmosphereHandler.java:154) ~[atmosphere-runtime-3.0.5.slf4jvaadin1.jar:3.0.5.slf4jvaadin1]
	at com.vaadin.flow.server.communication.PushAtmosphereHandler.onStateChange(PushAtmosphereHandler.java:54) ~[flow-server-24.8.0.beta2.jar:24.8.0.beta2]
	at org.atmosphere.cpr.DefaultBroadcaster.invokeOnStateChange(DefaultBroadcaster.java:1036) ~[atmosphere-runtime-3.0.5.slf4jvaadin1.jar:3.0.5.slf4jvaadin1]
	at org.atmosphere.cpr.DefaultBroadcaster.prepareInvokeOnStateChange(DefaultBroadcaster.java:1056) ~[atmosphere-runtime-3.0.5.slf4jvaadin1.jar:3.0.5.slf4jvaadin1]
	at org.atmosphere.cpr.DefaultBroadcaster.executeAsyncWrite(DefaultBroadcaster.java:870) ~[atmosphere-runtime-3.0.5.slf4jvaadin1.jar:3.0.5.slf4jvaadin1]
	at org.atmosphere.cpr.DefaultBroadcaster$2.run(DefaultBroadcaster.java:477) ~[atmosphere-runtime-3.0.5.slf4jvaadin1.jar:3.0.5.slf4jvaadin1]

Expected outcome

I would expect it to work like on Chrome.

Minimal reproducible example

private Anchor createDownloadLink() {
    return new Anchor(e -> {
        e.setFileName("myfile.txt");
        e.setContentType("text/plain");
        try {
            e.getOutputStream().write("AAAAAAAAAAAAAAAAAAAAAAAAA".getBytes(StandardCharsets.UTF_8));
        } catch (IOException ex) {
            throw new RuntimeException(ex);
        }
        e.getUI().access(() -> Notification.show("\"%s\" downloaded".formatted("myfile.txt")));
    }, "Download here");
}

Steps to reproduce

1. Use the code snipped
2. Click the download link.

Environment

Vaadin version(s): 24.8.0.beta2
OS: Linux Mint
Firefox: 139.0.0

Browsers

Firefox

[knoobie]

@mshabarov looks like your issue still persists (#21487)

[mshabarov]

Yes, this is the same issue.
The workaround for cases, when download a file is acceptable, is to add download attribute to the Anchor:

var anchor = new Anchor(event -> {...});
anchor.setDownload(true);

Ideally the problem with WS connection should be fixed.

[mshabarov]

It should now have the attribute by default via #21649.

[caalador]

For the WS connection issue there is an open bug on firefox that may be resolved at some point https://bugzilla.mozilla.org/show_bug.cgi?id=896666