ghostty-rs/docs/rewrite/DEVIANT_FIXES.md

Deviant fixes: where the Rust port intentionally differs from Ghostty 1.3.1

The rule

The port is faithful to Ghostty 1.3.1 (including its quirks), with one exception: genuine upstream bugs are fixed, not replicated. That means memory-safety problems and undefined behaviour (use-after-free, stale pointers, out-of-bounds reads, leaks), infinite loops, data loss or corruption (lost or overwritten output), and crashes/asserts reachable from input. See rule 2 in AGENT_GUIDE.md.

From now on every such fix is committed on its own as fix-deviant(<scope>): ... and gets a row in the first table below. This register also covers the deviations made before that convention; their commits bundle other work, so the "Commit" column names the commit that introduced the Rust behaviour (found with git log -S / git blame), not a dedicated fix commit.

Conventions: Zig paths are relative to ghostty/src/. Line numbers refer to the Zig tree used for the port. "unverified" means I could not confirm the detail from the notes or code; "none" means no test exercises the deviation. "uncommitted" means the change was in the working tree (agent Z17, ghostty-cli) when this file was written.

Upstream bugs fixed (behaviour deliberately differs)

#	Area	Zig location	Ghostty behaviour	Rust behaviour	Commit	Test
1	Kitty graphics: placement pins	terminal/kitty/graphics_storage.zig:148-176 (addPlacement), :517- (evictImage)	The tracked pin of a placement is leaked when its image is evicted and when an external placement id is reused (getOrPut then overwrite of the value).	add_image/add_placement/set_limit take the PageList and release the replaced/evicted placement's tracked pin.	d47408f	storage_add_placement_replaces_and_releases_pin, storage_eviction_prefers_unused_then_oldest (ghostty-terminal/src/kitty/graphics_storage.rs)
2	Kitty graphics: Placement::rect	terminal/kitty/graphics_storage.zig:~789-798 (grid_size.rows - 1, grid_size.cols - 1)	Unsigned underflow for a 0-sized grid.	saturating_sub(1) on rows/cols.	d47408f	none
3	Kitty graphics: file medium S=	terminal/kitty/graphics_image.zig:268-269 (appendRemaining(.limited(size)))	StreamTooLong whenever exactly size bytes are read, so any satisfiable S>0 fails with InvalidData (Zig tests only use S=0).	Data is truncated to min(S, 400MB), as the Kitty spec says.	d47408f (note clarified in 687752c)	image_load_file_offset_and_size (ghostty-terminal/src/kitty/graphics_image.rs)
4	Kitty graphics: zlib size boundary	terminal/kitty/graphics_image.zig:412 (.limited(max_size))	Decompression errors at exactly 400MB of output.	Errors only above 400MB (take(MAX_SIZE + 1) then len > MAX_SIZE).	d47408f	none
5	Renderer: link matching	renderer/link.zig:99-123 (defer offset = end)	A zero-length regex match leaves offset unchanged, so the loop never terminates.	A zero-length match advances by one character.	871c301	none dedicated (render_cell_map* in ghostty-renderer/src/link.rs do not cover it)
6	Input: keybind formatter	input/Binding.zig:2112-2150 (Set.Value.formatEntries)	The shared buffer is rewound to a fixed position after each sub-entry, which corrupts the prefix of sibling entries for chained leaves in sequences.	Each entry is formatted from its own prefix.	e034322	set_format_entries_sequence_shares_prefix (ghostty-input)
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)
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)
14	Renderer: GL Buffer::init_fill	renderer/opengl/buffer.zig:62	len = data.len * @sizeOf(T), so len is in bytes while the rest of the type counts elements.	len is the element count.	568ce78	none
15	Renderer: Pipeline teardown	renderer/opengl/Pipeline.zig:72-74	deinit destroys only the program; the FBO and VAO leak.	Drop also deletes the FBO and VAO.	568ce78	none
16	Renderer: custom-shader uniforms	renderer/generic.zig:2061-2064 (cursor_text.color, and the selection colours)	The union payload is read unconditionally; invalid for cell-foreground / cell-background.	Uniforms are set only for explicit colours.	871c301	none
17	Renderer (Metal): texture upload	renderer/metal/Texture.zig (replaceRegion)	The data length is not checked against the region.	A too-short slice is rejected (safe API). Zig line: unverified.	bab49ee	none
18	Input: binding order with catch-all	input/Binding.zig:287-294 (sort key of trigger.key)	Reads the key of a catch-all trigger from the C union, which is undefined.	A catch-all sorts as 0.	e034322	none
19	Font: unsupported FreeType pixel mode	font/face/freetype.zig:707	@panic("unsupported pixel mode").	Error::UnsupportedPixelMode.	2580176	none
20	Font: OpenType SVG table	font/opentype/svg.zig:45-46	Reads records[0] and the last record unconditionally, even for an empty document list.	An empty list returns an error.	2580176	none
21	Termio (Windows): process thread handle	Command.zig:347 (CreateProcessW)	The thread handle returned by CreateProcessW is never closed (leak).	It is closed.	76771be	none (compile-checked only on Windows)
22	Termio (Windows): focus_gained	termio/Exec.zig (Windows branch; exact line unverified)	@panic on Windows.	No-op.	76771be	none
23	GTK: close confirmation dialog (fallback build)	apprt/gtk/class/close_confirmation_dialog.zig:45-46	The fallback registers a signal named close-request, which already exists on GtkWindow; GObject aborts (dead code upstream).	The fallback names it dialog-close-request; the default build is unchanged.	f2252bb	runtime check recorded in notes/compat-GC.md; no unit test
24	libghostty: ghostty_surface_inherited_config	apprt/embedded.zig:1573, :941 (dupeZ of the working directory)	Allocates a new copy per call and never frees it.	The strings are kept in the surface and freed with it.	75bf604	none
25	libghostty: invalid enum values from C	apprt/embedded.zig (mouse state/button, split direction, colour scheme, key action, pressure stage)	Undefined behaviour or safety panic.	Ignored / returns false; null handles are tolerated.	75bf604	unverified
26	Crash reporting: envelope attachments	crash/sentry_envelope.zig:399-419 (Attachment.encode)	Never writes the type header, so the output cannot be parsed back.	Adds "type":"attachment" when absent.	b3145ad	envelope_parse_multiple (ghostty-crash/src/envelope.rs) is the nearest test; a dedicated test: unverified
27	Build data: webgen output	build/webgen/main_actions.zig:5-6, main_commands.zig:6, main_config.zig:6	A 2048-byte stdout buffer that is never flushed, so the generated website files are truncated.	The complete files are emitted.	e4cdd8e	none (checked byte-identical to Zig output of a patched copy, per final-Z4.md)
28	CLI: +list-colors plain output	cli/list_colors.zig:59-72	4096-byte buffer, never flushed; piped output stops midway through PeachPuff3 (12,297 bytes).	Flushed (15,434 bytes).	6286e02	none; found by RV11 (review-rv11-codex.md)
29	CLI: +list-themes diagnostics	cli/list_themes.zig:124-131,171	The messages ("Could not find the Ghostty resources directory", "No themes found...") go to a 4096-byte stderr buffer that is never flushed, so they are lost.	Flushed when the action ends.	410d8fc fix-deviant(cli)	zig_writer::tests::unflushed_buffer_is_flushed_on_drop
30	CLI: stdout as a regular file	std.fs.File.writer (Zig 0.15.2) as used in cli/show_face.zig:68-73, cli/help.zig:34, cli/validate_config.zig:43-46, cli/show_config.zig:84	The writer pwrites at its own offset starting at 0: with stdout redirected to a file, output overwrites itself and earlier file content, and Writer.end truncates.	Writes sequentially at the inherited descriptor's current offset (no pwrite from offset 0, so earlier content and the writers' own output are not overwritten; this appends only when the descriptor is opened for append, e.g. >>); no truncation.	410d8fc fix-deviant(cli) (append behaviour first in f76179f)	zig_writer::tests::regular_file_output_is_complete_and_appended
31	GTK: +new-window D-Bus arguments	apprt/gtk/ipc/new_window.zig:49-53 (Bytes.new(ptr, len + 1) + Variant.newFromBytes(..., trusted = true))	Arguments are passed as trusted GVariant strings without validation; invalid UTF-8 or an interior NUL makes GIO abort the process while serializing (input-triggered crash).	arguments_variant validates first and returns Error::IpcFailed (message on stderr, no D-Bus call); valid arguments are byte-identical to Zig.	65dd9d5 fix-deviant(gtk)	ipc::new_window::tests::invalid_dbus_strings_are_rejected_without_abort (ghostty-apprt-gtk)
32	OS: URL opener child reaping	os/open.zig (try collectOutput(...) before wait())	When collecting the opener's output fails (e.g. more than 50 KiB on a stream), the error returns before wait(), leaving the child as a zombie.	The child is always reaped; the collection error is still logged.	4a55abf fix-deviant(os)	ghostty-os open tests
33	OS: maximum-length NOTIFY_SOCKET	os/systemd.zig:100-116 (socket_address.path.len < socket_path.len, then path[socket_path.len] = 0)	A 108-byte path (the size of sockaddr_un.path) passes the length check, and the terminating NUL is written at path[108], one byte past the array (out-of-bounds write). Separate from row 13 (the abstract-socket @ handling).	The address is built with rustix's SocketAddrUnix::new, which takes the 108 bytes without a terminator (valid for Linux AF_UNIX) and rejects longer paths; no out-of-bounds write. Verified in code.	f8c5ce0 (git log -S on ghostty-os/src/systemd.rs: the only commit touching it, besides formatting/macOS changes)	notify_maximum_length_socket_path_is_rejected_safely (ghostty-os/src/systemd.rs)

Upstream quirks deliberately kept (not fixed)

These look odd but are not memory-safety, loop, corruption or crash bugs, so the port matches Ghostty.

• shadertoy::Uniforms: the selection foreground/background fields are swapped relative to the GLSL block. Both are vec3, so offsets agree (ghostty-renderer/src/shadertoy.rs:878; R1 note).
• Kitty storage: delete range uses || (matches everything); eviction stops only when evicted > req; set_limit(0) resets storage to defaults; screen reset also resets total_limit.
• lru::get does not change LRU order. Tabstops::unset XORs.
• Screen/keybind docs: format_entry_docs emits a blank line only (the @tagName lookup never matches).
• Search: an empty needle underflows exactly as in Zig, so the needle must be non-empty.
• i18n: the strlen == 7 guard skips the tables for 8-byte locale names.
• CSI: CSI K accepts 0..2 only; ANSI DECRQM is unimplemented; unknown many keycode media keys stay a TODO.
• libghostty C header: chostty_ipc_target_s typo kept; ghostty_inspector_metal_shutdown is bool in the header but void in Zig (we follow Zig).
• Braille sprites: debug_assert! where Zig asserts, so odd cell sizes (for example 14x23) trip it in builds with debug assertions, as in Zig.
• Windows: no file locking in the ssh-cache DiskCache (Zig does not lock on Windows either).
• Config theme-name basename and numeric parsing follow Zig 0.15.2, not 0.16 (fix(config) commits 3068e3a, a57ff41).

Other deliberate deviations (platform/Rust-imposed or out of scope)

Only entries with an observable behavioural difference are listed.

Area	Zig/Ghostty behaviour	Rust behaviour	Where	Commit
Swift macOS app	Ships macos/	Not ported; the Rust side provides the libghostty C API, Metal/CoreText code, XCFramework script. Only compile-checked (just check-macos), never run (no macOS here).	ARCHITECTURE.md (Out of scope), notes/macos-M1*.md, final-Z4.md	bab49ee, 75bf604, 231ee35
Windows (and wasm natively)	Native targets	Windows is compile-checked only (just check-windows), never run. wasm is built and its Node smoke/ABI tests run (just check-wasm, scripts/wasm-check.sh), but not in a real browser.	notes/windows-WIN.md, notes/wasm-W1.md	76771be, d89691b
Crash reporting	Sentry with Breakpad on macOS, SIGSEGV handlers, local DB/cache, sessions, init thread	In-process reporter: .ghosttycrash envelopes in $XDG_STATE_HOME/ghostty/crash, panic hook and signal handler, minidumps on macOS/Windows. No Sentry DB/cache dir, no sessions, no init thread; reports are written on any panic; the crash dir is resolved at init; the signal path writes no attachment.	notes/crash-CR.md, ARCHITECTURE.md (Scope changes)	b3145ad, 72bfac1
Surface copy-on-select on release	setSelection returns early when the new selection equals the old one	Left-release copies directly via copy_on_select, because the early return would never copy	notes/ghostty-core.md	ad76ae2
Locking / threading	Renderer mutex held while sending mailbox messages and running mouse reports (.locked sends); process_links called unlocked	Reports, IO messages and runtime actions are collected and run after the lock is released; process_links takes the lock itself; termio overflow goes to an ordered outbox	notes/ghostty-core.md, notes/ghostty-termio-IO*.md	6629fb0, ad76ae2
Rendering of overlay / images	z2d overlay, wuffs image decoding, glslang+SPIR-V+spirv-cross	Direct RGBA overlay with approximate hairline placement; png/jpeg-decoder; naga GLSL front end (narrower than glslang, with a sampler2D helper rewrite)	notes/ghostty-renderer-R1.md, R2.md, gaps-X2.md; just visual-parity reports 0 differing pixels for the tested scenes	568ce78, 871c301, f68b11d
Oniguruma	Native regex library for links and tmux	Hand-written matchers for the tmux parser; link regexes use the minimal ghostty-onig FFI crate	notes/ghostty-vt-W2.md	b2b248e, 2aa2a12
zlib / PNG / base64 / shm	zig std, wuffs, simdutf, mmap	flate2, png, base64 crate (ASCII whitespace ignored), rustix shm reads without mmap	notes/ghostty-terminal-kitty.md, notes/ghostty-vt-W2.md	d47408f, b2b248e
VT parser buffer	Fixed 2048-byte buffer with a reserved NUL	Growing Vec<u8> with a cap that models the reserved-NUL capacity; no observable difference is known (internal representation only)	notes/ghostty-vt-W1.md	b2b248e
CSI W logging	Logs a spurious "invalid cursor tabulation control" after CSI 2 W / CSI 5 W	No log line (log-only)	notes/ghostty-vt-stream.md	47b479d
Searching	CircBuf + overlap buffer, libxev thread	VecDeque, crossbeam-channel, 24 ms refresh timer; identical results, assertions about buffer layout dropped	notes/ghostty-terminal-search-tmux.md	8e9be42
Config strings	UTF-8 not required in many places	Escaped-bytes representation (EncodedOs) so non-UTF-8 survives; behaviour and error names matched to Zig (for example theme=/etc/passwd/x gives error.NotDir, checked against the oracle)	notes/review-rv11-codex.md (item 3)	672e8a4, 4bbe0d7
+version / build info	Real build metadata	Placeholder build_info values in places; gtk-opengl-debug follows cfg!(debug_assertions) (the release profile has them on, production off)	notes/ghostty-cli.md, notes/ghostty-config.md	6286e02
OSC 52 base64 acceptance	Surface.zig:2149,2163 rules	Was different in both directions (Codex core audit); fixed to match Zig	notes/review-core-codex.md	2f58f02
Benchmark / synthetic tooling extensions	ghostty-bench has no --time option and only the simplified terminal handler; ghostty-synthetic has the actions ascii, osc, utf8	ghostty-bench --time prints the measured step time to stderr; ghostty-bench --handler=full runs the full stream_readonly handler instead of the print-only one; ghostty-synthetic +sgr generates SGR-heavy output. Dev/benchmark tooling only; otherwise upstream behaviour is unchanged	crates/ghostty-bench/src/main.rs, src/benches.rs, src/bin/ghostty-synthetic.rs; notes/review-rv16-codex.md	0c4f0ee

Maintenance

When you fix a genuine upstream bug, commit it alone as fix-deviant(<scope>): ... and add a row to the first table with the hash, the Zig file:line and the test. Replace "uncommitted" entries with hashes once the Z17 work is committed.