fix(hashline): ignored deletes of the trailing newline sentinel in applyEdits

- Added a trailing-sentinel guard to identify newline-generated phantom lines and drop delete edits targeting them.
- Updated edit validation to allow those no-op deletes while preserving real line bounds checks.
- Adjusted format-v2 tests to expect no-op handling of sentinel deletes and clipping for ranges ending at the sentinel.

packages/hashline/src/apply.ts

@@ -35,26 +35,31 @@ function getEditAnchors(edit: AppliedEdit): Anchor[] {
 	return getCursorAnchors(edit.cursor);
 }
 
+function trailingPhantomLine(fileLines: readonly string[]): number {
+	// `split("\n")` on a newline-terminated file yields a trailing "" sentinel.
+	// It is addressable for inserts (append-past-end), but it is not real
+	// content. Deleting it only strips the file's final newline, so ignore delete
+	// edits that land there; inclusive ranges ending at EOF then do the intended
+	// thing and delete through the last concrete line.
+	return fileLines.length > 1 && fileLines[fileLines.length - 1] === "" ? fileLines.length : 0;
+}
+
+function dropTrailingPhantomDeletes(edits: AppliedEdit[], fileLines: readonly string[]): AppliedEdit[] {
+	const phantomLine = trailingPhantomLine(fileLines);
+	if (phantomLine === 0) return edits;
+	return edits.filter(edit => edit.kind !== "delete" || edit.anchor.line !== phantomLine);
+}
+
 /**
  * Verify every anchored edit points at an existing line. File-version binding is
  * checked once per section via the header hash before this function runs.
  */
-function validateLineBounds(edits: AppliedEdit[], fileLines: string[]): void {
-	// `split("\n")` on a newline-terminated file yields a trailing "" sentinel.
-	// It is addressable for inserts (append-past-end), but deleting it would
-	// silently strip the file's final newline — an off-by-one that must error.
-	const phantomLine = fileLines.length > 1 && fileLines[fileLines.length - 1] === "" ? fileLines.length : 0;
+function validateLineBounds(edits: readonly AppliedEdit[], fileLines: readonly string[]): void {
 	for (const edit of edits) {
 		for (const anchor of getEditAnchors(edit)) {
 			if (anchor.line < 1 || anchor.line > fileLines.length) {
 				throw new Error(`Line ${anchor.line} does not exist (file has ${fileLines.length} lines)`);
 			}
-			if (edit.kind === "delete" && anchor.line === phantomLine) {
-				throw new Error(
-					`Line ${anchor.line} is the trailing blank sentinel of a newline-terminated file and has no content to delete. ` +
-						`End the range at line ${anchor.line - 1}, or use \`insert tail:\` to append.`,
-				);
-			}
 		}
 	}
 }
@@ -742,7 +747,10 @@ export function applyEdits(text: string, edits: readonly Edit[]): ApplyResult {
 		if (firstChangedLine === undefined || line < firstChangedLine) firstChangedLine = line;
 	};
 
-	const targetEdits = appliedEdits.map((edit, index) => cloneAppliedEdit(edit, index));
+	const targetEdits = dropTrailingPhantomDeletes(
+		appliedEdits.map((edit, index) => cloneAppliedEdit(edit, index)),
+		fileLines,
+	);
 	validateLineBounds(targetEdits, fileLines);
 	const { edits: repaired, warnings: boundaryWarnings } = repairReplacementBoundaries(targetEdits, fileLines);
 	const { edits: landed, warnings: landingWarnings } = repairAfterInsertLandings(repaired, fileLines);

packages/hashline/test/format-v2.test.ts

@@ -66,15 +66,20 @@ describe("hashline format v4", () => {
 		expect(() => applyEdits("a\nb", edits)).toThrow(/Line 4 does not exist/);
 	});
 
-	it("rejects deleting the trailing blank sentinel of a newline-terminated file", () => {
+	it("ignores deleting the trailing blank sentinel of a newline-terminated file", () => {
 		// "a\nb\n" splits into ["a", "b", ""]; line 3 is the phantom sentinel.
 		const edits = parsePatch("delete 3").edits;
-		expect(() => applyEdits("a\nb\n", edits)).toThrow(/trailing blank sentinel/);
+		expect(applyEdits("a\nb\n", edits).text).toBe("a\nb\n");
 	});
 
-	it("rejects a replace range that spans the trailing blank sentinel", () => {
+	it("treats a delete range ending at the trailing sentinel as ending at the last real line", () => {
+		const edits = parsePatch("delete 2..3").edits;
+		expect(applyEdits("a\nb\n", edits).text).toBe("a\n");
+	});
+
+	it("treats a replace range ending at the trailing sentinel as ending at the last real line", () => {
 		const edits = parsePatch("replace 2..3:\n+B").edits;
-		expect(() => applyEdits("a\nb\n", edits)).toThrow(/trailing blank sentinel/);
+		expect(applyEdits("a\nb\n", edits).text).toBe("a\nB\n");
 	});
 
 	it("still allows inserts anchored on the trailing blank sentinel", () => {