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On Darwin a pty master has no line discipline until its slave has been
opened: every TIOCSWINSZ on the master until then fails with ENOTTY and
the size is dropped without a word. A pane is born 1x1 and told how big
it really is by the layout an instant after the fork -- and with the
child doing the open(), that message raced the child and usually lost.
Nothing ever sent it again, so the program in the pane spent its whole
life believing it had one row and one column. `C-a e` hit it every
time, because the editor's pane is always brand new: nvim drew its
screen into two rows in the corner of a 94x15 pane.
The old code knew half of this -- the child set the winsize on the
slave it had just opened, which is what kept a fresh pane from being
0x0 -- but that only fixed the *spawn* size. Every resize after it
still went to a master that might not have a slave yet.
So open the slave here, before the fork, and let the child inherit it.
The fork copies the fd table, so the slave is open from the child's
first instruction and every resize the parent sends afterwards lands;
BSD's forkpty() does the same thing for the same reason. The winsize is
set on that slave before the fork, which orders it ahead of anything
the parent sends later, and the child no longer sets it at all: by the
time the child ran, the size it was spawned with could already be
stale, and writing it back would undo the very resize this is about.
test_pty_size.py asks the pane's own program what size it thinks it is
-- for the first pane, for a pane from a split, and after a resize --
and compares it to the pane. It printed "1 1" against a 74x18 pane
before this.
live_m0.py asked `tput cols`, which needs a terminfo entry for
xterm-ghostty and therefore fails on most macs for a reason that has
nothing to do with resizing. `stty size` asks the tty itself, which is
the thing under test.
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