Fix backspace over a flag emoji leaving a dangling regional indicator

src/vs/base/common/strings.ts

@@ -1205,6 +1205,20 @@ function getOffsetBeforeLastEmojiComponent(initialOffset: number, str: string):
 
 	let resultOffset = iterator.offset;
 
+	if (isRegionalIndicator(codePoint)) {
+		// Regional indicators combine in pairs to form a single flag emoji
+		// (Unicode UAX #29 GB12/GB13: do not break within an emoji flag
+		// sequence). If this regional indicator completes such a pair,
+		// delete both code points together; otherwise it is a lone,
+		// unpaired indicator and only it should be deleted.
+		if (isRegionalIndicatorPaired(str, resultOffset)) {
+			const pairIterator = new CodePointIterator(str, resultOffset);
+			pairIterator.prevCodePoint();
+			resultOffset = pairIterator.offset;
+		}
+		return resultOffset;
+	}
+
 	if (resultOffset > 0) {
 		// Skip optional ZWJ code points that combine multiple emojis.
 		// In theory, we should check if that ZWJ actually combines multiple emojis
@@ -1222,6 +1236,27 @@ function isEmojiModifier(codePoint: number): boolean {
 	return 0x1F3FB <= codePoint && codePoint <= 0x1F3FF;
 }
 
+function isRegionalIndicator(codePoint: number): boolean {
+	return 0x1F1E6 <= codePoint && codePoint <= 0x1F1FF;
+}
+
+/**
+ * Returns whether the regional indicator ending at `offset` (already counted
+ * by the caller) is paired, i.e. whether the full contiguous run of regional
+ * indicators it belongs to has even length.
+ */
+function isRegionalIndicatorPaired(str: string, offset: number): boolean {
+	let runLength = 1;
+	const scan = new CodePointIterator(str, offset);
+	while (scan.offset > 0) {
+		if (!isRegionalIndicator(scan.prevCodePoint())) {
+			break;
+		}
+		runLength++;
+	}
+	return runLength % 2 === 0;
+}
+
 const enum CodePoint {
 	zwj = 0x200D,
 

src/vs/base/test/common/strings.test.ts

@@ -763,6 +763,51 @@ suite('Strings', () => {
 		assert.ok(strings.multibyteAwareBtoa(new Array(100000).fill('vs').join('')).length > 0); // https://github.com/microsoft/vscode/issues/112013
 	});
 
+	suite('getLeftDeleteOffset', () => {
+		function backspaceSteps(str: string): string[] {
+			const steps: string[] = [];
+			let s = str;
+			let offset = s.length;
+			while (offset > 0) {
+				const newOffset = strings.getLeftDeleteOffset(offset, s);
+				s = s.substring(0, newOffset) + s.substring(offset);
+				offset = newOffset;
+				steps.push(s);
+			}
+			return steps;
+		}
+
+		test('deletes an emoji modifier sequence as a single unit', () => {
+			// Skin tone modifier: baby + Fitzpatrick type-5 modifier.
+			assert.deepStrictEqual(backspaceSteps('\u{1F476}\u{1F3FE}'), ['']);
+		});
+
+		test('deletes a ZWJ emoji sequence component by component', () => {
+			// Family emoji: man ZWJ man ZWJ girl ZWJ girl.
+			assert.deepStrictEqual(
+				backspaceSteps('\u{1F468}\u{200D}\u{1F468}\u{200D}\u{1F467}\u{200D}\u{1F467}'),
+				['\u{1F468}\u{200D}\u{1F468}\u{200D}\u{1F467}', '\u{1F468}\u{200D}\u{1F468}', '\u{1F468}', '']
+			);
+		});
+
+		test('issue: deletes a complete flag emoji as a single unit, without leaving a dangling regional indicator', () => {
+			// Flag emoji are two regional indicator code points (UAX #29 GB12/GB13); they must not be split.
+			assert.deepStrictEqual(backspaceSteps('\u{1F1FA}\u{1F1F8}'), ['']); // 🇺🇸
+		});
+
+		test('issue: deletes adjacent flag emoji one flag at a time, not one indicator at a time', () => {
+			assert.deepStrictEqual(backspaceSteps('\u{1F1FA}\u{1F1F8}\u{1F1EC}\u{1F1E7}'), ['\u{1F1FA}\u{1F1F8}', '']); // 🇺🇸🇬🇧
+		});
+
+		test('deletes a trailing unpaired regional indicator alone, then the completed flag as a unit', () => {
+			assert.deepStrictEqual(backspaceSteps('\u{1F1FA}\u{1F1F8}\u{1F1EC}'), ['\u{1F1FA}\u{1F1F8}', '']); // 🇺🇸🇬
+		});
+
+		test('leaves non-emoji text untouched (single code point per backspace)', () => {
+			assert.deepStrictEqual(backspaceSteps('ab'), ['a', '']);
+		});
+	});
+
 	ensureNoDisposablesAreLeakedInTestSuite();
 });