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.
Astra
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