fix: align coverage diff columns

src/render.ts

@@ -77,12 +77,13 @@ function renderCoverageDiff(report: CoverageReport): string | null {
 
 	function fmtDiff(head: number, base: number): string {
 		const d = head - base;
-		if (d === 0) return "";
+		if (d === 0) return "0";
 		return d > 0 ? `+${d}` : String(d);
 	}
 
 	function fmtPctDiff(d: number | null): string {
-		if (d === null || d === 0) return "";
+		if (d === null) return "";
+		if (d === 0) return "0.00%";
 		return `${d > 0 ? "+" : ""}${d.toFixed(2)}%`;
 	}
 
@@ -124,14 +125,28 @@ function renderCoverageDiff(report: CoverageReport): string | null {
 
 	const actual = rows.filter((r): r is RowData => r !== "sep");
 	const lw = Math.max(...actual.map((r) => r.label.length));
-	const bw = Math.max(4, ...actual.map((r) => r.base.length));
-	const hw = Math.max(4, ...actual.map((r) => r.head.length));
-	const dw = Math.max(3, ...actual.map((r) => r.diff.length));
+	const bw = Math.max("base".length, ...actual.map((r) => r.base.length));
+	const hw = Math.max("head".length, ...actual.map((r) => r.head.length));
+	const dw = Math.max("+/-".length, ...actual.map((r) => r.diff.length));
+
+	function fmtColumns(
+		prefix: string,
+		label: string,
+		base: string,
+		head: string,
+		diff: string,
+	): string {
+		return `${prefix} ${label.padEnd(lw)}  ${base.padStart(bw)}  ${head.padStart(hw)}  ${
+			diff.padStart(dw)
+		}`;
+	}
 
 	function fmtRow(r: RowData): string {
-		return `${r.prefix} ${r.label.padEnd(lw)}  ${r.base.padStart(bw)}  ${
-			r.head.padStart(hw)
-		}  ${r.diff.padStart(dw)}`;
+		return fmtColumns(r.prefix, r.label, r.base, r.head, r.diff);
+	}
+
+	function fmtHeaderColumns(base: string, head: string, diff: string): string {
+		return `  ${"".padEnd(lw)}  ${base.padEnd(bw)}  ${head.padEnd(hw)}  ${diff.padEnd(dw)}`;
 	}
 
 	const rowLen = fmtRow(actual[0]).length;
@@ -145,9 +160,7 @@ function renderCoverageDiff(report: CoverageReport): string | null {
 	const tl = Math.floor(tPad / 2);
 	const tr = tPad - tl;
 
-	const colInner = `  ${" ".repeat(lw)}  ${"base".padStart(bw)}  ${"head".padStart(hw)}  ${
-		" +/-".padStart(dw)
-	}`;
+	const colInner = fmtHeaderColumns("base", "head", "+/-");
 
 	const lines: string[] = [
 		`@@${" ".repeat(tl)}${title}${" ".repeat(tr)}@@`,

src/render_test.ts

@@ -243,4 +243,64 @@ describe("renderReport", () => {
 		const hitsLine = diffBlock.split("\n").find((l) => l.includes("Hits"))!;
 		expect(hitsLine.startsWith("-")).toBe(true);
 	});
+
+	test("coverage diff table renders unchanged values explicitly", () => {
+		const head: FileCoverage[] = [
+			{ file: "a.ts", coveredLines: 7, totalLines: 10, percent: 70 },
+			{ file: "b.ts", coveredLines: 9, totalLines: 10, percent: 90 },
+		];
+
+		const base: CoverageArtifact = {
+			tool: "bun",
+			files: [
+				{ file: "a.ts", coveredLines: 7, totalLines: 10, percent: 70 },
+				{ file: "b.ts", coveredLines: 9, totalLines: 10, percent: 90 },
+			],
+			commitSha: "abc",
+			branch: "main",
+			timestamp: "2025-01-01T00:00:00Z",
+		};
+
+		const toolReport = buildToolReport("bun", head, base, []);
+		const fullReport = buildFullReport([toolReport]);
+		const md = renderReport(fullReport, "<!-- m -->", true);
+
+		const diffBlock = md.split("```diff")[1].split("```")[0];
+		expect(diffBlock).toContain("0.00%");
+		expect(diffBlock).toContain("Files");
+		expect(diffBlock).toContain("Lines");
+		expect(diffBlock).toContain("Hits");
+		expect(diffBlock).toMatch(/Files\s+2\s+2\s+0/);
+		expect(diffBlock).toMatch(/Lines\s+20\s+20\s+0/);
+		expect(diffBlock).toMatch(/Hits\s+16\s+16\s+0/);
+	});
+
+	test("coverage diff table header columns align with data columns", () => {
+		const head: FileCoverage[] = [
+			{ file: "src/index.ts", coveredLines: 8, totalLines: 10, percent: 80 },
+			{ file: "src/utils.ts", coveredLines: 10, totalLines: 10, percent: 100 },
+		];
+		const base: CoverageArtifact = {
+			tool: "bun",
+			files: [
+				{ file: "src/index.ts", coveredLines: 7, totalLines: 10, percent: 70 },
+				{ file: "src/utils.ts", coveredLines: 9, totalLines: 10, percent: 90 },
+			],
+			commitSha: "abc",
+			branch: "main",
+			timestamp: "2025-01-01T00:00:00Z",
+		};
+
+		const toolReport = buildToolReport("bun", head, base, []);
+		const fullReport = buildFullReport([toolReport]);
+		const md = renderReport(fullReport, "<!-- m -->", true);
+		const diffBlock = md.split("```diff")[1].split("```")[0];
+		const lines = diffBlock.trim().split("\n");
+		const headerLine = lines[1].slice(2, -2);
+		const coverageLine = lines[2];
+
+		expect(headerLine.indexOf("base")).toBe(coverageLine.indexOf("80.00%"));
+		expect(headerLine.indexOf("head")).toBe(coverageLine.indexOf("90.00%"));
+		expect(headerLine.indexOf("+/-")).toBe(coverageLine.indexOf("+10.00%"));
+	});
 });