While I can use Dillo without problems to connect to IPv4 HTTPS sites as
well as to IPv6 HTTP sites, connections to IPv6-only HTTPS sites appear
like stalled.

It seems as if dillo can cope well with IPv6, but its HTTPS helper
"dpid" can't. Given that HTTPS moves from the dpid into the browser with
the upcoming 3.1 version, I assume that this won't be fixed in the 3.0.x
series anymore.

I can reproduce this on Debian 8 Jessie (package 3.0.4-2+b1), Debian 9
Stretch (3.0.5-3), Debian 10 Buster (3.0.5-5) as well as on Debian Sid
as of now (package 3.0.5-5+b1).

Example hostnames to test:

Works: https://sym.noone.org/ (Dualstack + HTTPS)
Works: http://sym6.noone.org/ (IPv6-only)
Doesn't work: https://sym6.noone.org/ (IPv6-only + HTTPS)

(All three examples are the same host, but while sym.noone.org has an A
and AAAA record, sym6.noone.org only has a AAAA record for testing
purposes.)