gstack-post: a partial pr-body post keeps the PR's intro; only a full post owns the preamble

Co-authored-by: capy-ai[bot] <230910855+capy-ai[bot]@users.noreply.github.com>

TODOS.md

@@ -4,7 +4,6 @@
 
 ### P3: GST-421 and GST-412 follow-ups (filed 2026-10-31, from the PR B /ship review)
 
-- **Partial `gstack-post pr-body` posts drop the intro.** Since GST-412, any body file with a `## ` section replaces the text above the current body's first heading, so a lane that posts only `## Screenshots` deletes a human intro or a `Review-loop exception:` line written there (stderr names it; GitHub's edit history keeps it). Decide whether the preamble should be managed only when the file brings its own lead text or a receipt, or have /ship put the exception line under a named section. **Priority:** P3.
 - **Check the Approvals table's own cells.** `--check-prose` skips the whole rendered Approvals block, including the Asked, Covers and Still pending cells the lane types. Narrow the skip to the Exact words column. **Priority:** P3.
 - **The root `.bun/` blind spot.** `gstack-wtree` leaves every untracked file under a root `.bun/` out of the fingerprint, so an edit there never makes evidence STALE. Narrow the exclusion to Bun's cache subfolder if anything starts loading code from it. **Priority:** P3.
 

changelog.d/gsta195-prbody-intro.md

@@ -0,0 +1,7 @@
+**A partial `gstack-post pr-body` post keeps the PR's intro.** Since GST-412, any body file with a `## ` section replaced the text above the PR's first heading, so a lane that posted only `## Screenshots` deleted a human intro or a `Review-loop exception:` line under the receipt (GStack Minder: "a partial pr-body post must keep the human intro").
+
+### Fixed
+- **Only a full post owns the text above the first heading.** A file with a `## ` section and a receipt block or its own lead text, as /ship composes it, still replaces stale lead text and stale notes under the receipt. A file with `## ` sections only keeps them.
+
+### Tests
+- `test/gstack-post.test.ts`: a partial post keeps the intro and the note under the receipt; the full-post cases now carry a receipt, as /ship's body does.

docs/troubleshooting.md

@@ -2518,12 +2518,15 @@ same-titled section (title compared case-insensitively), the
 ````gstack-ship-receipt```` block is a section of its own so a receipt
 rewrite replaces only the receipt, sections the file does not rewrite stay in
 place, and new sections are appended after them. The text above the first
-`## ` heading belongs to the file whenever the file has a `## ` section: the
-current body's text there (the opening lines, and any lines between the receipt
-and the first heading, or the whole body when it has no `## ` heading) is
-replaced by the file's own text above its first heading, or dropped when the
-file has none. A file with no `## ` section (a receipt-only or text-only post)
-leaves that text alone. One stderr line names what was replaced, kept, added
+`## ` heading belongs to the file when the file is a full post: a `## `
+section plus a receipt block or text of its own above its first heading, as
+/ship composes it. Then the current body's text there (the opening lines, and
+any lines between the receipt and the first heading, or the whole body when it
+has no `## ` heading) is replaced by the file's own text above its first
+heading, or dropped when the file has none. A partial post (`## ` sections
+only, such as a lane posting `## Screenshots`) and a file with no `## `
+section (a receipt-only or text-only post) leave that text alone, so a human
+intro or a `Review-loop exception:` line under the receipt survives. One stderr line names what was replaced, kept, added
 and dropped, a dropped piece with its first line, or with that line withheld
 when the redaction scan flags it (`gstack-post: pr-body merged
 by section: replaced What changed; kept ship receipt, Why, Approvals; added

lib/gstack-post.ts

@@ -10,7 +10,8 @@
  * the same runner): a section in the file replaces the same-titled one, the
  * ship-receipt block is a section of its own, sections the file does not
  * rewrite are kept, new ones are appended, and a file with a `## ` section
- * replaces the text above the first heading; `--replace` overwrites instead.
+ * plus a receipt or its own lead text replaces the text above the first
+ * heading; `--replace` overwrites instead.
  * The scan runs on the merged bytes, which are the bytes sent.
  *
  * Exit codes: 0 posted; 1 HIGH finding (or oversize input) refused;
@@ -212,25 +213,27 @@ function splitSections(body: string): Section[] {
  * A file section whose title the current body holds more than once (an old
  * description pasted under the new one) matches no single section, so the
  * merge refuses instead of replacing one copy and keeping the stale other.
- * A file with at least one `## ` section manages the preamble (GST-412): every
+ * A full post (a file with at least one `## ` section and a receipt block or
+ * text of its own above its first heading) manages the preamble (GST-412): every
  * non-blank text section before the current body's first heading (the leading
  * text, text after a receipt block) is dropped, the file's own text before its
  * first heading takes its place (leading text first, text after the file's
  * receipt right after the receipt), a dropped piece the file brings no text for
  * is listed in `dropped` with its first line (withheld when the redaction scan
  * flags it, since the merged bytes no longer hold it), and the receipt keeps the rules
- * above. A file without a `## ` section merges as described above.
+ * above. A partial post (`## ` sections only) and a file without a `## `
+ * section merge as described above, so the current intro is kept.
  */
 export function mergeBodySections(current: string, incoming: string): { body: string; replaced: string[]; kept: string[]; added: string[]; dropped: string[] } {
   const now = splitSections(current);
   const next = splitSections(incoming).filter(n => !(n.key.startsWith("text:") && n.text.trim() === ""));
   const ambiguous = next.filter(n => now.filter(s => s.key === n.key).length > 1).map(n => `"${n.label}"`);
   if (ambiguous.length) throw new UsageError(`the PR's current body holds ${[...new Set(ambiguous)].join(", ")} more than once, so the file's copy matches no single section; nothing was posted. Rerun with --replace and the whole body`);
-  const managed = next.some(n => n.key.startsWith("h2:"));
   const preamble = (list: Section[]) => {
     const end = list.findIndex(s => s.key.startsWith("h2:"));
     return list.slice(0, end < 0 ? list.length : end).filter(s => s.key.startsWith("text:") && s.text.trim() !== "");
   };
+  const managed = next.some(n => n.key.startsWith("h2:")) && (next.some(n => n.key === "receipt") || preamble(next).length > 0);
   const stale = managed ? preamble(now) : [];
   const ours = managed ? preamble(next) : [];
   const role = (s: Section) => (s.key === "text:lead" ? "lead" : "trailing");

test/gstack-post.test.ts

@@ -457,10 +457,10 @@ describe("gstack-post pr-body merges the PR body by section", () => {
     expect(sent(stale).input).toBe(RECEIPT("bbbb") + "\n## What changed\n\n- new\n\n## Why\n\nBecause.\n\n## Approvals\n\n- owner: do it\n");
     expect(stale.stderr()).toBe("gstack-post: pr-body merged by section: replaced ship receipt, What changed; kept Why, Approvals; added none; dropped leading text (\"Draft intro from gh pr create, long enough to need truncati…\")\n");
 
-    const between = withBody(RECEIPT("aaaa") + "Old note under the receipt\n\n## What changed\n\n- old\n", { "/b": "## What changed\n\n- new\n" });
+    const between = withBody(RECEIPT("aaaa") + "Old note under the receipt\n\n## What changed\n\n- old\n", { "/b": RECEIPT("bbbb") + "## What changed\n\n- new\n" });
     expect(run(["pr-body", "5", "--body-file", "/b"], between)).toBe(EXIT.posted);
-    expect(sent(between).input).toBe(RECEIPT("aaaa") + "\n## What changed\n\n- new\n");
-    expect(between.stderr()).toBe("gstack-post: pr-body merged by section: replaced What changed; kept ship receipt; added none; dropped trailing text (\"Old note under the receipt\")\n");
+    expect(sent(between).input).toBe(RECEIPT("bbbb") + "\n## What changed\n\n- new\n");
+    expect(between.stderr()).toBe("gstack-post: pr-body merged by section: replaced ship receipt, What changed; kept none; added none; dropped trailing text (\"Old note under the receipt\")\n");
 
     const own = withBody("Stale intro\n\n" + RECEIPT("aaaa") + "Stale note\n\n## Why\n\nold\n", { "/b": "Fresh intro\n\n" + RECEIPT("bbbb") + "Fresh note\n\n## Why\n\nnew\n" });
     expect(run(["pr-body", "5", "--body-file", "/b"], own)).toBe(EXIT.posted);
@@ -469,6 +469,17 @@ describe("gstack-post pr-body merges the PR body by section", () => {
     expect(own.stderr()).toBe("gstack-post: pr-body merged by section: replaced leading text, ship receipt, trailing text, Why; kept none; added none\n");
   });
 
+  // Value: protects=a partial post (`## ` sections only, no receipt and no text of its own above its first heading) keeps the human intro and
+  //   the note under the receipt, the same as a full post that brings no replacement for them; fails_when=a lane posting only `## Screenshots`
+  //   deletes the intro or a Review-loop exception line; why_new=GST-412 made any file with a section own the preamble (GstackMinder,
+  //   2026-10-31: "a partial pr-body post must keep the human intro"); seam=fake run; no_claim=real gh
+  test("a partial post keeps the current intro and the note under the receipt", () => {
+    const partial = withBody("Human intro\n\n" + RECEIPT("aaaa") + "Review-loop exception: accepted by owner\n\n## What changed\n\n- old\n", { "/b": "## Screenshots\n\n![shot](u)\n" });
+    expect(run(["pr-body", "5", "--body-file", "/b"], partial)).toBe(EXIT.posted);
+    expect(sent(partial).input).toBe("Human intro\n\n" + RECEIPT("aaaa") + "Review-loop exception: accepted by owner\n\n## What changed\n\n- old\n\n## Screenshots\n\n![shot](u)\n");
+    expect(partial.stderr()).toBe("gstack-post: pr-body merged by section: replaced none; kept leading text, ship receipt, trailing text, What changed; added Screenshots\n");
+  });
+
   // Value: protects=the dropped-piece excerpt on stderr never repeats a secret the merge removed from the body (the scan only reads the merged bytes);
   // fails_when=a stale intro holding a credential is echoed to stderr while it leaves the PR; why_new=the excerpt is new in GST-412; seam=fake run; no_claim=real gh
   test("a dropped piece whose first line the redaction scan flags is named without its text", () => {
@@ -515,10 +526,10 @@ describe("gstack-post pr-body merges the PR body by section", () => {
     expect(replaced.stderr()).toBe("");
     expect(replaced.calls.some(c => c.cmd === "gh" && c.args[1] === "view")).toBe(false);
 
-    const plain = withBody("Just a paragraph.\nAnother line.", { "/b": "## Tests\n\n- one\n" });
+    const plain = withBody("Just a paragraph.\nAnother line.", { "/b": RECEIPT("bbbb") + "\n## Tests\n\n- one\n" });
     expect(run(["pr-body", "5", "--body-file", "/b"], plain)).toBe(EXIT.posted);
-    expect(sent(plain).input).toBe("## Tests\n\n- one\n");
-    expect(plain.stderr()).toBe("gstack-post: pr-body merged by section: replaced none; kept none; added Tests; dropped leading text (\"Just a paragraph.\")\n");
+    expect(sent(plain).input).toBe(RECEIPT("bbbb") + "\n## Tests\n\n- one\n");
+    expect(plain.stderr()).toBe("gstack-post: pr-body merged by section: replaced none; kept none; added ship receipt, Tests; dropped leading text (\"Just a paragraph.\")\n");
     const plainText = withBody("Just a paragraph.\nAnother line.", { "/b": RECEIPT("bbbb") });
     expect(run(["pr-body", "5", "--body-file", "/b"], plainText)).toBe(EXIT.posted);
     expect(sent(plainText).input).toBe(RECEIPT("bbbb") + "\nJust a paragraph.\nAnother line.");