New supergroups are created with "chat history for new members" hidden (the
client sets this right after creation, matching official Telegram). The
official server then forces it back to visible when the group is made public;
owpengram's UpdateUsername left the flag alone, leaving public groups in a
state where non-members (and post-join members) see no history at all.
UpdateUsername now clears pre_history_hidden whenever a non-empty username is
assigned, in the same transaction, with a matching admin-log event. Removing
the username leaves the flag untouched, so the creator can hide history again
once the group is private.
cachedParticipants returned a participant page with Hash=0 whenever the
channel_base / channel_participants rows in read_model_versions were never
seeded for a channel (e.g. groups created via messages.createChat). With
Hash=0 the RPC layer can never answer channels.channelParticipantsNotModified,
so a client that polls the member list re-fetches it in a tight loop forever.
Fall back to a deterministic content hash derived from the page itself
(channel id, page key, count, and each member's id/role/status/rank) so an
unchanged member list yields an identical non-zero Hash and the client
converges. The read-model-backed path is unchanged.
