@@ -2,7 +2,9 @@ import { describe, expect, test } from "bun:test";
 import { defined } from "../../testkit/defined.js";
 import {
   createMemoizedParseMarkdown,
+  nextStreamMarkdownState,
   parseMarkdown,
+  splitAtSettledBlock,
   splitAtSettledHeading,
   withholdIncompleteHeading,
   type StyledSegment,
@@ -567,3 +569,230 @@ describe("splitAtSettledHeading", () => {
     expect(splitAtSettledHeading("# Title")).toBeNull();
   });
 });
+
+describe("splitAtSettledBlock", () => {
+  test("freezes through the latest settled block (heading, fence, table)", () => {
+    const lines = [
+      "### Title",
+      "",
+      "```bash",
+      "echo hi",
+      "```",
+      "",
+      "| a | b |",
+      "|---|---|",
+      "| 1 | 2 |",
+      "",
+      "tail",
+    ];
+    expect(splitAtSettledBlock(lines.join("\n"))).toEqual({
+      frozen: lines.slice(0, 9).join("\n"),
+      live: "tail",
+      gapRows: 1,
+    });
+  });
+
+  test("freezes a closed fence with prose after the closer", () => {
+    expect(splitAtSettledBlock("```bash\necho hi\n```\n\ntail")).toEqual({
+      frozen: "```bash\necho hi\n```",
+      live: "tail",
+      gapRows: 1,
+    });
+  });
+
+  test("supports tilde fences and indented openers up to three spaces", () => {
+    expect(splitAtSettledBlock("~~~\ncode\n~~~\n\ntail")).toEqual({
+      frozen: "~~~\ncode\n~~~",
+      live: "tail",
+      gapRows: 1,
+    });
+    expect(splitAtSettledBlock("  ```js\ncode\n  ```\n\ntail")).toEqual({
+      frozen: "  ```js\ncode\n  ```",
+      live: "tail",
+      gapRows: 1,
+    });
+  });
+
+  test("a four-space indented fence is not a fence, so earlier text can settle", () => {
+    expect(splitAtSettledBlock("## T\n\n    ```\nx")).toEqual({
+      frozen: "## T",
+      live: "    ```\nx",
+      gapRows: 1,
+    });
+  });
+
+  test("an unclosed fence after settled text still leaves the settled split in place", () => {
+    expect(splitAtSettledBlock("### T\n\n```bash\necho hi")).toEqual({
+      frozen: "### T",
+      live: "```bash\necho hi",
+      gapRows: 1,
+    });
+    expect(splitAtSettledBlock("```bash\necho hi")).toBeNull();
+  });
+
+  test("a closed block at the last line falls back to the earlier split", () => {
+    expect(splitAtSettledBlock("## T\n\n```\ncode\n```")).toEqual({
+      frozen: "## T",
+      live: "```\ncode\n```",
+      gapRows: 1,
+    });
+    expect(splitAtSettledBlock("## T\n\n| h |\n|---|\n| r |")).toEqual({
+      frozen: "## T",
+      live: "| h |\n|---|\n| r |",
+      gapRows: 1,
+    });
+    expect(splitAtSettledBlock("```\ncode\n```")).toBeNull();
+    expect(splitAtSettledBlock("| h |\n|---|\n| r |")).toBeNull();
+  });
+
+  test("freezes through a completed separator table, not just the heading", () => {
+    const lines = [
+      "## T",
+      "",
+      "Name | Age",
+      "--- | ---",
+      "Ada | 36",
+      "",
+      "tail",
+    ];
+    expect(splitAtSettledBlock(lines.join("\n"))).toEqual({
+      frozen: lines.slice(0, 5).join("\n"),
+      live: "tail",
+      gapRows: 1,
+    });
+  });
+
+  test("freezes through a completed borderless table", () => {
+    const lines = [
+      "## T",
+      "",
+      "| h1 | h2 |",
+      "| r1 | r2 |",
+      "| r3 | r4 |",
+      "",
+      "tail",
+    ];
+    expect(splitAtSettledBlock(lines.join("\n"))).toEqual({
+      frozen: lines.slice(0, 5).join("\n"),
+      live: "tail",
+      gapRows: 1,
+    });
+  });
+
+  test("a table that reaches the last line keeps the row live", () => {
+    expect(splitAtSettledBlock("| h |\n|---|\n| r1 |\n| r2")).toBeNull();
+    expect(splitAtSettledBlock("## T\n\n| h |\n|---|\n| r1 |\n| r2")).toEqual({
+      frozen: "## T",
+      live: "| h |\n|---|\n| r1 |\n| r2",
+      gapRows: 1,
+    });
+  });
+
+  test("a table with no separator is not a table", () => {
+    expect(splitAtSettledBlock("a | b\n\ntail")).toBeNull();
+  });
+
+  test("agrees with splitAtSettledHeading on heading-only content", () => {
+    for (const text of ["# Title\nbody", "# Title", "plain"]) {
+      expect(splitAtSettledBlock(text)).toEqual(splitAtSettledHeading(text));
+    }
+  });
+});
+
+describe("nextStreamMarkdownState", () => {
+  const frozen = "### T";
+  const base = {
+    content: `${frozen}\n\ntail`,
+    frozen,
+    width: 100,
+    streaming: true,
+  };
+
+  test("a first paint always repaints", () => {
+    expect(
+      nextStreamMarkdownState(
+        null,
+        base.content,
+        base.frozen,
+        base.width,
+        true,
+      ),
+    ).toEqual({
+      state: base,
+      paintFrozen: true,
+    });
+  });
+
+  test("tail-only growth skips the frozen repaint", () => {
+    const grown = { ...base, content: `${frozen}\n\ntail grows` };
+    const next = nextStreamMarkdownState(
+      base,
+      grown.content,
+      grown.frozen,
+      grown.width,
+      true,
+    );
+    expect(next.paintFrozen).toBe(false);
+    expect(next.state.content).toBe(grown.content);
+  });
+
+  test("a moved boundary repaints the frozen node", () => {
+    const moved = { ...base, frozen: `${frozen}\n\nsettled` };
+    expect(
+      nextStreamMarkdownState(
+        base,
+        base.content,
+        moved.frozen,
+        base.width,
+        true,
+      ).paintFrozen,
+    ).toBe(true);
+  });
+
+  test("width and streaming flips repaint the frozen node", () => {
+    expect(
+      nextStreamMarkdownState(
+        base,
+        base.content,
+        base.frozen,
+        base.width + 1,
+        true,
+      ).paintFrozen,
+    ).toBe(true);
+    expect(
+      nextStreamMarkdownState(
+        base,
+        base.content,
+        base.frozen,
+        base.width,
+        false,
+      ).paintFrozen,
+    ).toBe(true);
+  });
+
+  test("mid-stream edits repaint the frozen node", () => {
+    const edited = `${frozen}\n\nrewritten`;
+    const prev = { ...base, content: `${frozen}\n\ntail` };
+    const next = nextStreamMarkdownState(
+      prev,
+      edited,
+      frozen,
+      base.width,
+      true,
+    );
+    expect(next.paintFrozen).toBe(true);
+    expect(next.state).toBeDefined();
+  });
+
+  test("identical content still skips the frozen repaint", () => {
+    const next = nextStreamMarkdownState(
+      base,
+      base.content,
+      base.frozen,
+      base.width,
+      true,
+    );
+    expect(next.paintFrozen).toBe(false);
+    expect(next.state).toEqual(base);
+  });
+});
@@ -854,3 +854,119 @@ export function splitAtSettledHeading(text: string): MarkdownSplit | null {
     gapRows: firstContent > 0 ? 1 : 0,
   };
 }
+
+/**
+ * A markdown body split at the last settled block boundary: a settled ATX
+ * heading line, the closing fence of a completed fenced block, or the last
+ * row of a table the renderer would consume. Returns null when no boundary
+ * has settled yet (or when the only boundary is the last line, leaving no
+ * live tail).
+ *
+ * The walk below mirrors parseMarkdown's dispatch order line for line —
+ * loose fence pairing (`FENCE_OPEN_RE`/`FENCE_CLOSE_RE`), indented-code runs,
+ * then `parseTableBlock`'s start/consume/shape rules — so every recorded
+ * boundary is a block boundary in the one-shot parse, and the frozen prefix
+ * plus the live suffix render the same apart as they do together. An
+ * unclosed fence consumes to the end of the row, so nothing after its opener
+ * can settle. A single non-blank line between a boundary and the live tail
+ * still yields `gapRows: 0`; blank lines collapse to one gap row, as before.
+ */
+export function splitAtSettledBlock(withheld: string): MarkdownSplit | null {
+  const lines = withheld.split("\n");
+  const boundaries = new Set<number>();
+  let i = 0;
+  while (i < lines.length) {
+    const line = lines[i] ?? "";
+    if (FENCE_OPEN_RE.test(line)) {
+      let closer = -1;
+      for (let j = i + 1; j < lines.length; j += 1) {
+        if (FENCE_CLOSE_RE.test(lines[j] ?? "")) {
+          closer = j;
+          break;
+        }
+      }
+      if (closer === -1) break;
+      boundaries.add(closer);
+      i = closer + 1;
+      continue;
+    }
+    if (INDENTED_CODE_RE.test(line)) {
+      i += 1;
+      while (i < lines.length && INDENTED_CODE_RE.test(lines[i] ?? "")) i += 1;
+      continue;
+    }
+    if (looksLikeTableRow(line)) {
+      const next = lines[i + 1];
+      const hasSeparator = next !== undefined && isTableSeparator(next);
+      const rawRows: string[][] = [extractTableCells(line)];
+      let consumed = 1;
+      if (hasSeparator) {
+        rawRows.push(extractTableCells(next ?? ""));
+        consumed = 2;
+      }
+      while (i + consumed < lines.length) {
+        const rowLine = lines[i + consumed] ?? "";
+        if (!looksLikeTableRow(rowLine) || isTableSeparator(rowLine)) break;
+        rawRows.push(extractTableCells(rowLine));
+        consumed += 1;
+      }
+      const firstWidth = rawRows[0]?.length ?? 0;
+      const valid =
+        hasSeparator ||
+        (rawRows.length >= 2 &&
+          firstWidth >= 2 &&
+          rawRows.every((row) => row.length === firstWidth));
+      if (valid) {
+        boundaries.add(i + consumed - 1);
+        i += consumed;
+        continue;
+      }
+    }
+    if (ATX_HEADING_LINE_RE.test(line)) boundaries.add(i);
+    i += 1;
+  }
+  const ordered = [...boundaries].sort((a, b) => b - a);
+  for (const boundary of ordered) {
+    if (boundary >= lines.length - 1) continue;
+    const rest = lines.slice(boundary + 1);
+    const firstContent = rest.findIndex((entry) => entry.trim().length > 0);
+    if (firstContent === -1) continue;
+    return {
+      frozen: lines.slice(0, boundary + 1).join("\n"),
+      live: rest.slice(firstContent).join("\n"),
+      gapRows: firstContent > 0 ? 1 : 0,
+    };
+  }
+  return null;
+}
+
+/** Paint state for one streaming markdown row across repaints. */
+export interface StreamMarkdownSnapshot {
+  readonly content: string;
+  readonly frozen: string | null;
+  readonly width: number;
+  readonly streaming: boolean;
+}
+
+/**
+ * Decide whether a streaming markdown repaint must repaint the frozen node.
+ * The same frozen text at the same width, still streaming a pure append,
+ * needs no frozen repaint; a moved boundary, a resize, a streaming-flag
+ * flip, or a non-append edit (rollback, rewrite) repaints it.
+ */
+export function nextStreamMarkdownState(
+  prev: StreamMarkdownSnapshot | null,
+  content: string,
+  frozen: string | null,
+  width: number,
+  streaming: boolean,
+): { state: StreamMarkdownSnapshot; paintFrozen: boolean } {
+  const state: StreamMarkdownSnapshot = { content, frozen, width, streaming };
+  if (prev === null) return { state, paintFrozen: true };
+  const paintFrozen =
+    prev.frozen !== frozen ||
+    prev.width !== width ||
+    prev.streaming !== streaming ||
+    !content.startsWith(prev.content);
+  return { state, paintFrozen };
+}