CI coverage flagged MarkdownText's pre route: the array-element probe
(raw[0]) and the mixed-content fallbacks were unreachable — the markdown
pipeline hands pre one code element whose children are one string (or
none, for an empty fence). Simplify to the string check, annotate the
isValidElement guard as representation-change armor, and pin both live
arms: the empty fence keeps the stock <pre>, a language-less fence renders
the plain CodeBlock arm.
ds-review-bot finding: a growing fence retokenized on every chunk
(quadratic main-thread work). MarkdownText gains a streaming flag —
the streaming partial renders fences through the plain arm and the
finalize swap highlights once; AssistantMarkdown threads its existing
flag. (The zh Agent Note pair the review also flagged landed earlier
on this branch.) New spec pins plain-while-streaming and
highlighted-after-finalize.
One highlighter for the client: a synchronous fine-grained shiki core
(JS regex engine, no WASM) in ui-primitives with an explicit grammar
allowlist (typescript, shellscript, json — aliases resolve, unknown
languages take a geometry-identical plain arm). The shared CodeBlock
component owns both arms; markdown fences, the run_code expanded
program body (typescript), and the details panel Input (json) all
route through it. Token colors live in a new ui-theme shiki.css sheet
as --shiki-* custom properties (light/dark blocks), wired through the
shell's base.css chain — tokens-only styling holds; shiki's generated
span tree is the sanctioned innerHTML path (static output, no user
HTML). jsdom specs pin token spans, aliases, both fallbacks, and the
fence route; the built-bundle snapshot asserts the highlighted program
under the code row.
