fix(tui): handle Thai Sara Am width

closes #3904

packages/tui/CHANGELOG.md

@@ -2,6 +2,10 @@
 
 ## [Unreleased]
 
+### Fixed
+
+- Fixed editor rendering artifacts for Thai Sara Am and Lao AM vowel characters ([#3904](https://github.com/badlogic/pi-mono/issues/3904))
+
 ## [0.70.6] - 2026-04-28
 
 ## [0.70.5] - 2026-04-27

packages/tui/src/utils.ts

@@ -178,14 +178,26 @@ function graphemeWidth(segment: string): number {
 		return 2;
 	}
 
+	// Thai SARA AM (U+0E33) and Lao VOWEL SIGN AM (U+0EB3) are encoded as
+	// spacing letters, but terminals commonly render isolated occurrences as
+	// mark-like clusters (compat decomposition: nonspacing niggahita + vowel).
+	// Count isolated clusters conservatively to avoid cursor drift and stale
+	// cells during differential rendering. When attached to a base consonant,
+	// the base code point is first and normal per-codepoint width below applies.
+	if (cp === 0x0e33 || cp === 0x0eb3) {
+		return 2;
+	}
+
 	let width = eastAsianWidth(cp);
 
-	// Trailing halfwidth/fullwidth forms
+	// Trailing halfwidth/fullwidth forms and AM vowels that segment with a base.
 	if (segment.length > 1) {
 		for (const char of segment.slice(1)) {
 			const c = char.codePointAt(0)!;
 			if (c >= 0xff00 && c <= 0xffef) {
 				width += eastAsianWidth(c);
+			} else if (c === 0x0e33 || c === 0x0eb3) {
+				width += 1;
 			}
 		}
 	}

packages/tui/test/editor.test.ts

@@ -626,6 +626,18 @@ describe("Editor component", () => {
 			}
 		});
 
+		it("renders isolated Thai and Lao AM clusters without width drift", () => {
+			for (const text of ["ำabc", "ຳabc"]) {
+				const editor = new Editor(createTestTUI(), defaultEditorTheme);
+				const width = 8;
+				editor.setText(text);
+
+				for (const line of editor.render(width)) {
+					assert.strictEqual(visibleWidth(line), width, `line width drift for ${JSON.stringify(text)}: ${line}`);
+				}
+			}
+		});
+
 		it("wraps CJK characters correctly (each is 2 columns wide)", () => {
 			const editor = new Editor(createTestTUI(), defaultEditorTheme);
 			const width = 10 + 1; // +1 col reserved for cursor

packages/tui/test/truncate-to-width.test.ts

@@ -59,4 +59,11 @@ describe("visibleWidth", () => {
 	it("counts tabs inline and skips ANSI inline", () => {
 		assert.strictEqual(visibleWidth("\t\x1b[31m界\x1b[0m"), 5);
 	});
+
+	it("counts isolated Thai and Lao AM clusters conservatively", () => {
+		assert.strictEqual(visibleWidth("ำ"), 2);
+		assert.strictEqual(visibleWidth("ຳ"), 2);
+		assert.strictEqual(visibleWidth("กำ"), 2);
+		assert.strictEqual(visibleWidth("ກຳ"), 2);
+	});
 });