UseIpv6=1 appears to disable IPv4 connectivity instead of enabling dual-stack IPv4/IPv6

[sgw03407]

Hello UltraVNC maintainers,

I am a Japanese UltraVNC user and have been using UltraVNC for almost 15 years.
I sincerely appreciate your continuous maintenance and development of this project.

My English is limited, so I am posting this using AI-assisted translation. Please excuse any unnatural wording.

While testing UltraVNC with IPv6 enabled, I noticed behavior that may be unexpected.

When UseIpv6=1 is set in ultravnc.ini, IPv4 clients appear to be unable to connect to winvnc.
When UseIpv6=0, IPv4 connections work as expected.

I would like to ask whether this is the intended behavior.

From a user's point of view, the option name "UseIpv6" may be interpreted as enabling IPv6 in addition to IPv4, rather than switching the server to IPv6-only behavior.

If the current behavior is intentional, perhaps documenting it as IPv6-only would help avoid confusion.
If it is not intentional, dual-stack IPv4/IPv6 support might be worth considering.

Thank you again for your work on UltraVNC.