Fix TUI tab width accounting

closes #5218

packages/tui/CHANGELOG.md

@@ -2,6 +2,10 @@
 
 ## [Unreleased]
 
+### Fixed
+
+- Fixed tab width accounting in column slicing and overlay compositing so tab-containing output cannot exceed the terminal width ([#5218](https://github.com/earendil-works/pi/issues/5218)).
+
 ## [0.78.0] - 2026-05-29
 
 ### Fixed

packages/tui/src/utils.ts

@@ -162,6 +162,10 @@ function finalizeTruncatedResult(
  * check to avoid running the RGI_Emoji regex unnecessarily.
  */
 function graphemeWidth(segment: string): number {
+	if (segment === "\t") {
+		return 3;
+	}
+
 	// Zero-width clusters
 	if (zeroWidthRegex.test(segment)) {
 		return 0;

packages/tui/test/tab-width.test.ts

@@ -0,0 +1,23 @@
+import assert from "node:assert";
+import { describe, it } from "node:test";
+import { extractSegments, sliceWithWidth, visibleWidth } from "../src/utils.ts";
+
+describe("tab width accounting", () => {
+	it("keeps slice helper widths consistent with visible width", () => {
+		const text = "out 192M\t.pi/skill-tests/results-ha";
+		const slice = sliceWithWidth(text, 0, 10, true);
+
+		assert.strictEqual(slice.text, "out 192M");
+		assert.strictEqual(slice.width, 8);
+		assert.strictEqual(visibleWidth(slice.text), slice.width);
+	});
+
+	it("keeps overlay segment widths consistent with visible width", () => {
+		const text = "out 192M\t.pi/skill-tests/results-ha";
+		const segments = extractSegments(text, 10, 13, 10, true);
+
+		assert.strictEqual(segments.before, "out 192M\t");
+		assert.strictEqual(segments.beforeWidth, 11);
+		assert.strictEqual(visibleWidth(segments.before), segments.beforeWidth);
+	});
+});