fix(app): keep diff rows aligned for blank lines

packages/app/src/utils/diff-rendering.test.ts

@@ -0,0 +1,23 @@
+import { describe, expect, it } from "vitest";
+
+import { formatDiffContentText, formatDiffGutterText, hasVisibleDiffTokens } from "./diff-rendering";
+
+describe("diff-rendering", () => {
+  it("keeps header gutters tall even when they do not show a line number", () => {
+    expect(formatDiffGutterText(null)).toBe(" ");
+    expect(formatDiffGutterText(82)).toBe("82");
+  });
+
+  it("keeps empty split cells tall even when they have no visible content", () => {
+    expect(formatDiffContentText(undefined)).toBe(" ");
+    expect(formatDiffContentText("")).toBe(" ");
+    expect(formatDiffContentText("const value = 1;")).toBe("const value = 1;");
+  });
+
+  it("treats empty highlighted token rows as blank lines instead of visible content", () => {
+    expect(hasVisibleDiffTokens(undefined)).toBe(false);
+    expect(hasVisibleDiffTokens([])).toBe(false);
+    expect(hasVisibleDiffTokens([{ text: "" }])).toBe(false);
+    expect(hasVisibleDiffTokens([{ text: "const value = 1;" }])).toBe(true);
+  });
+});