[Help-bash] History always appended even though histappend set to off

[john smith]

When I start bash with --norc histappend is set to off as expected:

$ shopt | grep histappend
histappend      off

After starting bash with --norc in two different terminal windows I
expect only history of the last closed window to be saved.
However, I always end up with history from both windows saved in
~/.bash_history after exiting as if histappend was set.  I checked
the source code and the reason for this behavior is in
maybe_save_shell_history() in this line:

if (history_lines_this_session <= where_history () || force_append_history)

The first condition is always true.  I added printing value of
history_lines_this_session and where_history() before exitting:

$ ./bash --norc
bash-4.3$ echo hi
hi
bash-4.3$ exit
exit
Will append
history_lines_this_session: 2
where_history(): 6

What is going on here?  In what circumstances does histappend work?

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