fix-deviant(terminal): no panics on input-reachable terminal states

Intentional deviation from Ghostty 1.3.1 (DEVIANT_FIXES.md entry 10).
Two terminal inputs reach states that Zig handles only by assertion
(Debug/ReleaseSafe abort; ReleaseFast continues):
- Toggling mode 2027 around a BOM before a wide character hits
  `assert(!b)` in print_grapheme. This repository's release profile
  keeps debug assertions, so terminal output could crash a release
  build. The assertion is removed and the path behaves like Zig
  ReleaseFast.
- A grapheme (e.g. a Devanagari conjunct) at the right margin on the
  bottom row with scrolling margins returned InvalidCoordinate and left
  a grapheme flag without data, panicking later in the formatter. The
  grapheme now moves from the cell that actually holds it and no cell is
  flagged without data.

tools/difftest gains a Rust-only panic hunt (`difftest_fuzz panics`,
lenient driver): ~60k debug-build cases now run without any panic, and
3000 Zig differential cases show 0 divergences.

Test Plan:
- cargo test -p ghostty-terminal difftest_mode_2027_toggle_with_bom_does_not_panic
  difftest_grapheme_wrap_scrolling_margins_keeps_grapheme
- cargo run -p difftest --bin difftest_fuzz -- panics

crates/ghostty-terminal/src/stream_readonly.rs

@@ -1078,6 +1078,31 @@ mod tests {
         assert!(!screen_row_wrap(&t, 1));
     }
 
+    #[test]
+    fn difftest_grapheme_wrap_scrolling_margins_keeps_grapheme() {
+        // Same Devanagari conjunct at the right margin, but on the bottom row
+        // so the wrap scrolls the margin region and the grapheme data travels
+        // with its row. Zig asserts/faults (`moveGrapheme` on a blank cell);
+        // Rust must neither error nor leave a grapheme flag without data.
+        let mut t = term(255, 5);
+        t.vt_stream()
+            .next_slice("\x1b[?2027h\x1b[?69h\x1b[;9s\x1b[24H\t\u{915}\u{94d}\u{937}".as_bytes())
+            .unwrap();
+        let text = t.plain_string().unwrap();
+        assert!(text.contains('\u{915}'));
+    }
+
+    #[test]
+    fn difftest_mode_2027_toggle_with_bom_does_not_panic() {
+        // Zig asserts `!graphemeBreak` while replaying an existing grapheme
+        // (aborts in safe builds, ignored in ReleaseFast). It must never panic
+        // here: this repository's release profile enables debug assertions.
+        let mut t = term(20, 8);
+        t.vt_stream()
+            .next_slice(b"\x1b[?2027la\xef\xbb\xbf\x1b[?2027h\xe4\xb8\xad")
+            .unwrap();
+    }
+
     #[test]
     fn difftest_grapheme_wrap_with_margins_moves_prev_cell() {
         // Devanagari conjunct widening the cell at the last column while a

crates/ghostty-terminal/src/terminal.rs

@@ -820,8 +820,12 @@ impl Terminal {
                     .expect("prev cell");
                 let cps = page.lookup_grapheme(idx).expect("grapheme data");
                 for &cp2 in cps {
-                    let b = ghostty_unicode::grapheme_break(cp1, cp2, &mut state);
+                    // Zig asserts `!graphemeBreak(cp1, cp2)` here, but that
-                    debug_assert!(!b);
+                    // is reachable from input (mode 2027 toggled after the
+                    // grapheme was built, e.g. with a BOM), so it must not
+                    // panic. ReleaseFast ignores the result; we do too. This
+                    // only advances `state`.
+                    let _ = ghostty_unicode::grapheme_break(cp1, cp2, &mut state);
                     cp1 = cp2;
                 }
             }
@@ -943,18 +947,31 @@ impl Terminal {
                                 let new_idx = page.cell_index(new_x, new_y).expect("new cell");
                                 // Zig moves from `prev.cell`, NOT from `old_pin`
                                 // (they differ when margins keep the row from
-                                // scrolling the cell out of `old_pin`).
+                                // scrolling the cell out of `old_pin`). When
-                                let old_idx = prev_idx;
+                                // the wrap DID scroll the margin region, the
+                                // data travelled with its row to `old_pin` and
+                                // `prev.cell` is blank: Zig then asserts
+                                // (page.zig moveGrapheme) or faults in
+                                // ReleaseFast. Move from where the data really
+                                // is, and never flag a cell without data.
+                                let old_idx = if page.cells[prev_idx].has_grapheme() {
+                                    prev_idx
+                                } else {
+                                    page.cell_index(old_x, old_y).expect("old cell")
+                                };
+                                if page.cells[old_idx].has_grapheme() {
                                     page.move_grapheme(old_idx, new_idx)?;
                                     page.cells[old_idx].set_content_tag(ContentTag::Codepoint);
-                                page.cells[new_idx].set_content_tag(ContentTag::CodepointGrapheme);
+                                    page.cells[new_idx]
+                                        .set_content_tag(ContentTag::CodepointGrapheme);
                                     page.rows[new_y].set_grapheme(true);
+                                }
                                 page.update_row_grapheme_flag(old_y);
                             } else {
                                 let cps: Vec<u32> = {
                                     let page = self.screen().page(old_pin.node);
                                     let old_idx = page.cell_index(old_x, old_y).expect("old cell");
-                                    page.lookup_grapheme(old_idx).expect("grapheme").to_vec()
+                                    page.lookup_grapheme(old_idx).unwrap_or_default().to_vec()
                                 };
                                 for cp in cps {
                                     let x = self.screen().cursor.x;

docs/rewrite/DEVIANT_FIXES.md

@@ -35,7 +35,7 @@ when this file was written.
 | 7 | Core: selection replacement | `Surface.zig:2310-2322` (`setSelection`) | `select()` untracks the previous selection's pins, then `sel.eql(prev)` compares through those released pins (stale pointers). | The comparison happens before the old selection is replaced. (Rust handles are checked, so the naive port panicked; the e2e tests found it.) | 2f58f02 | `select_all_twice_with_copy_on_select_does_not_panic` (`ghostty-core/tests/e2e_bindings.rs`), `drag_selects_and_copy_on_select_fills_selection_clipboard` (`e2e_mouse.rs`). Dedicated eql-before-replace test: unverified |
 | 8 | Datastruct: `lru` eviction report | `datastruct/lru.zig:163-166` | `entry.data.key` is overwritten with the new key before `entry.data` is copied out as `evicted`, so the evicted key is wrong. | `GetOrPutResult.evicted` carries the old key and old value. | 5b8c92b | `get_or_put` checks only `evicted.value`; the key is not asserted (`ghostty-datastruct/src/lru.rs`) |
 | 9 | Terminal: stale cell pointer in grapheme attach | `terminal/Terminal.zig:482` and `:539` | `prev.cell` survives a page reallocation (`increaseCapacity` in `printCell` of the spacer tail), so `appendGrapheme` writes into freed memory and the grapheme (for example U+FE0F) is lost. | Cells are addressed by index; the grapheme is kept. | 4eb03f6 | `difftest_upstream_uaf_grapheme_survives_page_growth` (`ghostty-terminal/src/stream_readonly.rs`); the difftest harness stores such mismatches as `suspect*` |
-| 10 | Terminal: asserts and segfaults reachable from input | `Terminal.zig:375` (`assert(!graphemeBreak)` after mode 2027 with U+2028/U+2060/BOM), `terminal/page.zig:1012` (`assert(y < size.rows)`; Devanagari conjuncts at the right edge with margins), `Terminal.zig:834` (`assert(!cell.hyperlink)`; REP with a huge count while a hyperlinked page grows) | Zig aborts or segfaults (roughly 0.1-0.4 % of generated difftest cases). | Rust handles the input without panicking. Zig-aborting cases are excluded from the differential comparison, not treated as divergences. | 4eb03f6 | `difftest_grapheme_wrap_with_margins_moves_prev_cell` and the harness (`tools/difftest`, `ZIG CRASH` handling) |
+| 10 | Terminal: asserts and segfaults reachable from input | `Terminal.zig:375` (`assert(!unicode.graphemeBreak(cp1, cp2, &state))` after mode 2027 is toggled with U+2028/U+2060/BOM), `terminal/page.zig:1012` (`assert(y < size.rows)`; Devanagari conjunct at the right margin, including the bottom-row case where the margin wrap scrolls and `moveGrapheme(prev.cell, ...)` runs on a blank cell), `Terminal.zig:834` (`assert(!cell.hyperlink)`; REP with a huge count while a hyperlinked page grows) | Zig aborts (ReleaseSafe) or faults (ReleaseFast) in the margin case; the first and third abort only with assertions on (roughly 0.1-0.4 % of generated difftest cases). | No panic and no error. The grapheme replay ignores the break result like ReleaseFast (the assertion is dropped, because this repository's `release` profile enables debug assertions, so input could crash a release build). The margin case moves the grapheme from the cell that actually holds it (`prev.cell`, else the cell above) and never flags a cell without data. Verified with a Rust-only panic hunt (`difftest_fuzz panics`, debug build, about 60k cases, 0 panics) and the Zig differential run (0 divergences). Zig-aborting cases are excluded from the differential comparison. | 4eb03f6 (REP, first margin case); Z21 (uncommitted): assertion and bottom-row margin case | `difftest_grapheme_wrap_with_margins_moves_prev_cell`, `difftest_grapheme_wrap_scrolling_margins_keeps_grapheme`, `difftest_mode_2027_toggle_with_bom_does_not_panic`, `tools/difftest` (`ZIG CRASH` handling, `panics` mode) |
 | 11 | VT: tmux control-mode ids | `terminal/tmux/control.zig:199,230,270,308` (`parseInt(usize) catch unreachable`) | An id that overflows `usize` hits `unreachable` (crash/UB). | The line is treated as a failed match. | b2b248e | none |
 | 12 | VT: SGR separator bitset | `terminal/sgr.zig:228,514` (`params_sep.isSet(idx)` past `MAX_PARAMS`) | Asserts / out-of-range in `StaticBitSet` for long parameter lists. | Bounds-tolerant `sep_is_set`. | b2b248e | `sgr_underline_colon_with_trailing_separator_and_short_slice` (`ghostty-vt/src/sgr.rs`); whether Zig's path is reachable: unverified |
 | 13 | OS: `NOTIFY_SOCKET` | `os/systemd.zig:100-116` | A leading `@` (abstract socket) is accepted by the check, then copied literally into the address, which cannot work. | A leading `@` becomes an abstract-namespace address. | f8c5ce0 | none (the notify tests use a path socket) |

tools/difftest/src/bin/difftest_fuzz.rs

@@ -4,6 +4,7 @@
 //! difftest_fuzz run [--seed N] [--cases N] [--jobs N] [--tokens N]
 //!                   [--zig PATH] [--work DIR] [--size CxR] [--sb BYTES]
 //! difftest_fuzz minz|minr <cols> <rows> <sb> <input-file> # minimise an abort
+//! difftest_fuzz panics [--seed N] [--cases N] [--tokens N]  # Rust only, no Zig
 //! difftest_fuzz min <cols> <rows> <sb> <input-file>   # minimise a divergence
 //! difftest_fuzz check <cols> <rows> <sb> <input-file> # compare one input
 //! ```
@@ -168,6 +169,10 @@ fn main() {
     let work = root.join("target/difftest-work");
     std::fs::create_dir_all(&work).expect("mkdir");
     let ctx = Arc::new(Ctx { zig, work });
+    if a.first().map(String::as_str) == Some("panics") {
+        panics(&a, &ctx);
+        return;
+    }
     std::panic::set_hook(Box::new(|_| {}));
     match a.first().map(String::as_str) {
         Some("check" | "min" | "minz" | "minr") => {
@@ -305,3 +310,43 @@ fn run(ctx: &Arc<Ctx>, a: &[String]) {
         std::process::exit(1);
     }
 }
+
+fn lenient_panics(input: &[u8], c: u16, r: u16, sb: usize) -> bool {
+    let input = input.to_vec();
+    std::panic::catch_unwind(move || ghostty_difftest::drive_lenient(&input, c, r, sb)).is_err()
+}
+
+/// Rust-only panic hunt (run it with a debug build so assertions are on):
+/// untrusted terminal input must never panic, even where Zig aborts.
+fn panics(a: &[String], ctx: &Ctx) {
+    let seed: u64 = arg(a, "--seed", 1);
+    let cases: usize = arg(a, "--cases", 1000);
+    let tokens: usize = arg(a, "--tokens", 60);
+    let mut found = 0;
+    for i in 0..cases {
+        let mut rng = Rng(seed.wrapping_mul(0x1_0000_0001).wrapping_add(i as u64));
+        let (c, r, sb) = config(&mut rng);
+        let n = 1 + rng.below(tokens);
+        let input = generate(&mut rng, n);
+        if lenient_panics(&input, c, r, sb) {
+            found += 1;
+            let mut min = input.clone();
+            let mut i = 0;
+            while i < min.len() {
+                let mut cand = min.clone();
+                cand.remove(i);
+                if lenient_panics(&cand, c, r, sb) {
+                    min = cand;
+                } else {
+                    i += 1;
+                }
+            }
+            let _ = ctx;
+            eprintln!("case {i}: {c}x{r} sb={sb} minimised={min:02x?}");
+        }
+    }
+    println!("seed={seed} cases={cases} rust panics={found}");
+    if found > 0 {
+        std::process::exit(1);
+    }
+}

tools/difftest/src/lib.rs

@@ -200,6 +200,16 @@ pub fn dump(input: &[u8], cols: u16, rows: u16, max_scrollback: usize) -> String
     dump_terminal(&t)
 }
 
+/// Feed `input` and ignore stream errors (they are reported to the caller as
+/// `Err`, not a crash). Used by the Rust-only panic hunt.
+pub fn drive_lenient(input: &[u8], cols: u16, rows: u16, max_scrollback: usize) -> String {
+    let mut opts = Options::new(cols, rows);
+    opts.max_scrollback = max_scrollback;
+    let mut t = Terminal::new(opts).expect("terminal");
+    let _ = t.vt_stream().next_slice(input);
+    dump_terminal(&t)
+}
+
 /// Canonical dump of an already-driven terminal.
 pub fn dump_terminal(t: &Terminal) -> String {
     let mut w = String::new();