4c - the guard's spread was a class: the code block's held lines and a mark run's segments too

src/markdown/emit/adf-to-markdown.test.ts

@@ -736,3 +736,8 @@ test('carries whitespace CommonMark strips in the reserved text directive', () =
   // CommonMark strips spaces and tabs alone, so the whitespace beside them is plain text.
   assert.equal(emitted({ text: '\va\f', type: 'text' }), '\va\f\n')
 })
+
+test("joins a mark run's segments as a walk rather than as one call's arguments", () => {
+  const run = Array.from({ length: 200000 }, (): AdfNode => ({ marks: [{ type: 'strong' }], text: 'a', type: 'text' }))
+  assert.equal(markdown(adfToMarkdown(document({ content: run, type: 'paragraph' }))), `**${'a'.repeat(200000)}**\n`)
+})

src/markdown/emit/inline-line.ts

@@ -166,7 +166,7 @@ function emitRun(nodes: readonly AdfNode[], depth: number, firstIndex: number, c
     const emitted = run.kind === 'plain' ? emitLeaf(run.node, runContext, run.index) : emitMarkedRun(run.nodes, run.mark, depth, run.index, runContext)
     if (!emitted.ok) return emitted
     if (emitted.value.carry !== undefined) return emitted
-    segments.push(...emitted.value.segments)
+    for (const segment of emitted.value.segments) segments.push(segment)
   }
   return success({ segments })
 }