Backends assume an interactive terminal: non-TTY stdout, NO_COLOR, TERM=dumb, SIGTERM

[mellonis]

• Packages: @flowtty/tty-backend, @flowtty/inline-tty-backend, render() in @flowtty/react — v1.0.0-alpha.11
• Area: robustness outside an interactive terminal

TtyBackend assumes it is talking to a capable, interactive terminal and never checks. Each item below was reproduced with a five-line app (render(<Text color="red" bold>hello</Text>, new TtyBackend())).

Observed

• stdout is not a TTY (mycli | tee log, CI, a file): the alt-screen and cursor sequences and every SGR code are written into the pipe — the log starts with ESC[?1049h ESC[?25l ESC[2J ESC[H …. Nothing in the backend looks at stdout.isTTY (only stdin's, for raw mode).
• NO_COLOR=1 is ignored: ESC[1;31m is still emitted.
• TERM=dumb is ignored: the alt screen is still entered.
• SIGTERM / SIGHUP: the process dies with the terminal left in the alt screen and the cursor hidden — neither ESC[?1049l nor ESC[?25h is written. Only Ctrl-C / Ctrl-D (read as keys in raw mode) and uncaught errors restore it; render() installs no signal handlers. Same for bracketed paste and mouse reporting, which stay switched on.

Not checked yet

• Windows Terminal / ConPTY: key sequences, alt screen, resize.
• stdin is not a TTY while stdout is (input piped in): onKey silently never fires.
• Terminals without truecolor: rgb() / #hex are always emitted as 24-bit SGR, with no 256-color or 16-color downgrade.

Expected — to decide per item, roughly

• Non-TTY stdout: either refuse with a clear error, or degrade to a plain, final-frame text dump without control sequences (a render option; the inline backend's printStatic path is close to this already).
• Honor NO_COLOR (drop color SGR, keep bold/dim/underline) and FORCE_COLOR.
• TERM=dumb: treat like a non-TTY.
• Restore the terminal on SIGTERM / SIGHUP / SIGINT, then re-raise, so the exit status is still the signal's.
• A README section stating what happens in each of these cases.

Value: clear. This is the class of failure a user meets on day one — a garbled CI log, a terminal stuck in the alt screen — and none of it needs new features.

[mellonis]

Done on a branch (not released yet):

• SIGTERM / SIGHUP / SIGINT restore the terminal, then re-raise — exit status 143 / 129 / 130, checked against a real process.
• NO_COLOR / FORCE_COLOR honored; bold/dim/underline/inverse survive; both backends take a color option.
• stdout is not a terminal / TERM=dumb: TtyBackend refuses with a clear error before writing a byte; InlineTtyBackend prints its <Static> lines as plain text and skips the live region (logOnly). isInteractive() is exported for branching. A test runs the inline example as a real piped process.

Still open: Windows Terminal / ConPTY, stdin not a TTY while stdout is, and a 256 / 16-color downgrade for terminals without truecolor.

[mellonis]

The truecolor downgrade moved to its own issue, #26. What remains here needs an environment to check it in: Windows Terminal / ConPTY, and stdin not being a TTY while stdout is.