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ddidderr 6befc09cc8 fix: pixel-identical rendering (z2d stroker port) and last P3 items

Resolves the 3 P3 items in docs/rewrite/notes/review-rv12-codex.md
(final-Z15.md):
- Sprite strokes: tiny-skia is replaced by a port of z2d 0.10's stroke
  plotter (sprite/stroke.rs) plus z2d's arc segmentation and truncating
  integer compositing, feeding the ported 4x4 multisample rasterizer.
  All 36 reference sprite sheets now match Ghostty's byte for byte (test
  tolerance 0), and `just visual-parity` with a production build shows
  0 differing pixels (max delta 0) against the installed Ghostty 1.3.1
  for the test page (block and bar cursor) and the tabs+splits scenario,
  terminal area and chrome alike. tiny-skia dependency removed.
- zig_error maps errno values per target via libc (Linux, Darwin/BSD)
  and Win32 codes the way Zig std does on Windows.
- theme=. / theme=.. report "it is a directory" via a port of
  std.fs.path.basename, matching Ghostty.

Test Plan:
- cargo test --workspace --no-fail-fast (3498 passed, exit 0)
- cargo build --profile production && just visual-parity
- just check-macos (both), just check-windows, just check-wasm

2026-10-12 14:20:49 +02:00

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Visual parity vs original Ghostty 1.3.1 (agent VP)

Method (`scripts/visual-parity.sh`, `just visual-parity [scenario...]`)

A private headless weston (--backend=headless --renderer=gl, 1200x800, desktop-shell, no panel) per run, on its own socket in $XDG_RUNTIME_DIR=/tmp/vp-runtime-$UID (socket paths must be <108 bytes). The user's desktop is never touched. Mesa is forced for weston (__EGL_VENDOR_LIBRARY_FILENAMES -> 50_mesa.json); the NVIDIA EGL crashes Xwayland's glamor. XWAYLAND_NO_GLAMOR=1.
weston --debug is what authorizes weston-screenshooter (without it: "Output capture error: unauthorized").
Both apps run with env -i, GSETTINGS_BACKEND=memory, no D-Bus, isolated XDG_CONFIG_HOME with the same explicit config (JetBrains Mono 13, bg/fg, padding 8, maximize = true so the window geometry is deterministic, cursor-style-blink = false, title = Ghostty, no shell integration) and LANG=C.UTF-8 (otherwise printf '\U..' in the test page prints literally).
Scenarios: page (block cursor), page-bar (bar cursor), tabs (new tab + split right + split down, driven with xdotool through Xwayland since the original Ghostty has no input replay; Rust is driven the same way so both apps are treated identically). Test page: scripts/visual-parity/testpage.sh (ASCII, ligatures, bold/italic/faint/inverse/strike/ overline, all underline styles + colored, 16/256/truecolor, box drawing, blocks, braille, powerline, nerd font icons, CJK, emoji, wide chars, math, cursor). Not covered: mouse selection (no input injection into the original on Wayland; could be added through the Xwayland path).
scripts/visual-parity/compare.py: per-pixel max-channel delta, % differing, % > 8 / > 32 levels, max, mean, red heat-map PNG; run on the full frame, the terminal area and the headerbar strip. scripts/visual-parity/zoom.py: ref | rust | diff crops, upscaled.
Output (not committed): target/visual-parity/<scenario>/{ref,rust,diff,diff-term,diff-chrome}.png, target/visual-parity/stats.txt. RUST_BIN defaults to target/production/ghostty. Iteration build: CARGO_TARGET_DIR=target/vp CARGO_PROFILE_PRODUCTION_LTO=off CARGO_PROFILE_PRODUCTION_CODEGEN_UNITS=16 CARGO_PROFILE_PRODUCTION_INCREMENTAL=true cargo build --profile production.

Results (% of pixels differing at all; max channel delta)

scenario	before	after
page (full frame)	0.130% (1250 px, max 93)	0.000% (0 px; was 0.020% / 196 px before Z15)
page-bar	same as page	0.000%
tabs + splits	44.6% (a whole pane blank, bug)	0.000% (bit-identical)
chrome (headerbar, tab bar)	0.000%	0.000%

Sprite sheets (ghostty-font tests, Zig reference PNGs): differing pixels 5.76% -> 1.20% -> 0 (bit-exact after Z15; test tolerance is now zero).

Font metrics, cell size, baseline, glyph rasterization, colors, cursor, underline positions, padding, GTK CSS/headerbar/tab bar were already pixel-identical; no change was needed there.

Fixes

Split rebuild blanked an unchanged pane (ghostty-core/src/surface/mod.rs, draw_frame): the GTK render handler called renderer.draw_frame(false); Zig's glRender uses drawFrame(true). With sync = false and nothing dirty the renderer only re-presents the last target, which is cleared when the GLArea is re-realized (nested split = tree rebuild = unrealize/realize of every surface whose size did not change), so that pane stayed empty until something changed. Now true like Zig.
Sprite antialiasing (ghostty-font/src/sprite/canvas.rs): replaced tiny-skia's coverage with a port of z2d 0.10's default multisample_4x rasterizer (4x4 samples, scanline spans with edge x rounded at sample row centers, non-zero filter, alpha = coverage * 16 - 1, full = 255, z2d's Bezier flattening with tolerance 0.1). (Stroke outlines were still tiny-skia's until Z15, which ported z2d's stroker.)

Remaining differences

None (Z15). z2d's stroke plotter (pen, joins, caps, spline flattening), Cairo-style arc segmentation and z2d's truncating integer compositing were ported (ghostty-font/src/sprite/stroke.rs, canvas.rs; see final-Z15.md). Final just visual-parity: page, page-bar and tabs are 0 differing pixels on the full frame, the terminal area and the chrome (max delta 0). All sprite sheets in the ghostty-font reference test are bit-exact (tolerance now zero).

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