From e1d210e20ca5bea3e5f8b48731d565e2ea2b04e2 Mon Sep 17 00:00:00 2001
From: Kit Langton <kit.langton@gmail.com>
Date: Fri, 6 Nov 2026 17:18:07 -0800
Subject: [PATCH] fix(core): keep trailing blank lines a patch does not touch

derive() split the file into lines, dropped the terminating empty element, and
then restored a final newline only when the last line was not already empty.
A file ending in a blank line therefore lost that blank line on every update,
and a hunk adding an empty line at the end of a file had it silently dropped.
Always restoring the terminator preserves the file's own trailing newlines
while still adding one to a file that lacked it.
---
 packages/core/test/patch.test.ts | 9 +++++++++
 packages/util/src/patch.ts       | 5 ++++-
 2 files changed, 13 insertions(+), 1 deletion(-)

diff --git a/packages/core/test/patch.test.ts b/packages/core/test/patch.test.ts
index 22df2a13271..9c8577f1bf1 100644
--- a/packages/core/test/patch.test.ts
+++ b/packages/core/test/patch.test.ts
@@ -248,6 +248,15 @@ describe("Patch", () => {
     ).toBe("line 1\nline 2\nadded 1\nadded 2\n")
   })
 
+  test("keeps trailing blank lines the patch did not touch", () => {
+    expect(
+      Patch.derive("update.txt", [{ oldLines: ["line 2"], newLines: ["LINE 2"] }], "line 1\nline 2\n\n").content,
+    ).toBe("line 1\nLINE 2\n\n")
+    expect(
+      Patch.derive("update.txt", [{ oldLines: ["line 1"], newLines: ["line 1", ""] }], "line 1\n").content,
+    ).toBe("line 1\n\n")
+  })
+
   test.each(["", "original\n"])("preserves equal-offset insertion order and frozen chunks for %j", (original) => {
     const chunks = Object.freeze([
       Object.freeze({ oldLines: Object.freeze([]), newLines: Object.freeze(["first"]) }),
diff --git a/packages/util/src/patch.ts b/packages/util/src/patch.ts
index cef1c3132da..c0583c66352 100644
--- a/packages/util/src/patch.ts
+++ b/packages/util/src/patch.ts
@@ -131,7 +131,10 @@ export function derive(path: string, chunks: ReadonlyArray<UpdateFileChunk>, ori
   if (lines.at(-1) === "") lines.pop()
   const replacements = computeReplacements(lines, path, chunks)
   for (const [start, remove, insert] of replacements.reverse()) lines.splice(start, remove, ...insert)
-  if (lines.at(-1) !== "") lines.push("")
+  // The split above lost the final newline as an empty element, and the file must end with one. Always
+  // restoring it keeps any other trailing blank lines the patch did not touch, which is why this is not
+  // conditional on the array's last line.
+  lines.push("")
   const next = Bom.split(lines.join("\n"))
   return { content: next.text, bom: source.bom || next.bom }
 }