sl0ppty: one-shot --headless printed a blank screen

`--headless` is documented as "run once, print the screen", and it printed
nothing for any command that finished — which is most of them. `make smoke`
had been showing six blank rows.

settle() calls app_reap() every iteration, and app_reap closes a pane whose
child has exited; closing the last one takes the tab with it. A command like
`sh -c printf ...` exits immediately, so the pane and its content were gone
before app_compose ever ran, and the compose found an app with no tabs.

So settle() now takes an optional screen and composes into it before the reap
rather than after. This is exact rather than a narrowed race: pane_pump() only
marks a pane dead on read() returning 0, and it reaches EOF only after it has
drained everything the pty had, so a pane cannot be marked dead while its
output is still pending and the last snapshot always holds the finished
screen. --script passes NULL and is byte-for-byte unchanged.

smoke also asked for 6 rows, which after the outer margin, the tab strip and
two borders leaves about one row for the pane — under min_pane rows, so it
collapsed to an empty frame and would have shown nothing even with the fix.
It asks for 12 now.

Found while renaming; it predates the rename, reproduced on the commit before.

1 files changed, 1 insertions, 1 deletions

diff --git a/Makefile b/Makefile
index dc614d4..88f0f3d 100644
--- a/Makefile
+++ b/Makefile
@@ -70,7 +70,7 @@ test-live: $(BIN) ## the checks that need a real tty (slow)
 test-all: test test-live ## everything
 
 smoke: $(BIN) ## compose a screen headlessly and print it
-	./$(BIN) --headless --cols 40 --rows 6 -- /bin/sh -c 'printf "hello \033[1;32msl0ppty\033[0m\n"; echo "wide: 日本語"'
+	./$(BIN) --headless --cols 40 --rows 12 -- /bin/sh -c 'printf "hello \033[1;32msl0ppty\033[0m\n"; echo "wide: 日本語"'
 
 run: $(BIN) ## build and run
 	./$(BIN)