The last T4 prerequisite, and the one that had to be a ruling rather than a
packet: a canvas, an exporter, a hit test, and an accessibility tree must agree
about text, and what they agree on is decided by where the shaper runs relative
to the canonical boundary.
The census reframes the tranche. Three of the five categories the plan names --
lyrics, chord symbols, rehearsal marks -- carry no text in the model at all:
LyricLine holds only event references, ChordSymbol and Marker only an anchor.
They are blocked on a core-track schema major, not on this decision. What the
primitive does gate is the text the model already has: score metadata,
instrument and staff names, and text-line spanners. That is a smaller v1 than
the plan implied and a real one, and bidi and fallback are exercised through
synthetic fixtures that need no model work.
The ruling is a fourth resolved primitive carrying the source string and the
canonical shaped result together. The alternative that discards the string
renders a title as anonymous outlines and is unreadable to a screen reader; the
alternative that discards the shaped result lets two consumers draw the same
bytes differently, which contradicts the definition of canonical_bytes as the
rendering fingerprint. Both halves stay, and the apparent trade between
deterministic geometry and accessibility turns out not to exist.
Two drafts were wrong in opposite directions and the errors are recorded rather
than quietly fixed, because each came from asserting a constraint instead of
reading the requirement that governs it. Draft 1 held that shaping before the
canonical boundary poisons cross-implementation byte equality -- but layout
determinism is byte-equal only within one implementation at a fixed version;
across implementations it is reference-suite thresholds, and the spec says so in
both the determinism table and req:solver:cross-implementation-conformance.
Draft 1 had imported the score layer's guarantee into the layout layer, where
the spec deliberately weakens it. Revision 2 then over-corrected, banning host
fonts outright on the grounds that an OS font update breaks fixed-version
stability -- but that requirement defines identical inputs to include font
metrics referenced by version and content hash, so an updated font is a changed
input. The rule that survives is narrower than either: no ambient or unresolved
lookup, and a host face may participate only once resolved to an exact
content-hashed asset every consumer can obtain.
The identity is specified rather than gestured at, because bytes that do not
determine ink are worse than bytes that admit they don't. A face is pinned by a
hash over the font file, not its metrics -- GlyphCatalogIdentity's metrics_hash
covers bounding boxes, advances and anchors, which pins spacing and not shape --
together with face index, variation coordinates and synthetic weight/slant.
Segments carry font-internal glyph ids, source ranges, direction, script,
language and em size; glyph offsets have alignment already applied, so a
consumer places by origin alone; positions quantize on the same 1/1024 grid as
every other primitive. The cluster map indexes UTF-8 byte offsets with caret
stops at grapheme boundaries carrying bidi affinity, and the Unicode
segmentation version is always part of the identity -- otherwise two
implementations could agree on every pixel and still differ inside the
fingerprint, where no visual test would ever see it.
One consequence lands on the exporter: SVG cannot honour "no consumer reshapes"
with <text>, which carries characters and lets the viewer's shaper choose the
glyphs, so a ligature or positional form silently draws something the layout did
not resolve. Conformant text export emits explicit glyphs as paths through the
same face, reusing the mode render-svg already has for music.
The reservation is re-ordered to follow shaping rather than precede it -- with a
canonical shaper in the pipeline, reserved_box becomes a solver policy over
measured bounds, not an estimate of them. Paint-time re-spacing stays forbidden.
Two findings for the core track, named so their absence is a decision. Score
text authored through operations is not NFC-validated: the envelope's NFC-
checked string reader covers only directly encoded strings such as transaction
labels, while SetMetadataOp, CreateStaffOp, CreateInstrumentOp and the cross-
cutting values embed the core codec's bytes, which preserve non-NFC strings by
design. And the .tex amendment adding the primitive changes the layout
fingerprint but needs no bundle or wire schema-major move, following strokes and
curves.
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01RSX4zSLgKvtiXaPjnMqLGz
Levi Neuwirth
f639919ee1