fix off by one issue, #16277

src/vs/editor/contrib/snippet/common/snippetController.ts

@@ -377,12 +377,12 @@ class BeforeAfterData {
 
 		let contentBefore = '';
 		if (overwriteBefore > 0) {
-			contentBefore = model.getLineContent(selection.startLineNumber).substring(selection.startColumn - 1 - overwriteBefore, selection.startColumn);
+			contentBefore = model.getLineContent(selection.startLineNumber).substring(selection.startColumn - 1 - overwriteBefore, selection.startColumn - 1);
 		}
 
 		let contentAfter = '';
 		if (overwriteAfter > 0) {
-			contentAfter = model.getLineContent(selection.endLineNumber).substring(selection.endColumn - 1, selection.endColumn + overwriteAfter);
+			contentAfter = model.getLineContent(selection.endLineNumber).substring(selection.endColumn - 1, selection.endColumn - 1 + overwriteAfter);
 		}
 
 		return new BeforeAfterData(model, contentBefore, contentAfter, overwriteBefore, overwriteAfter);

src/vs/editor/contrib/snippet/test/common/snippetController.test.ts

@@ -506,4 +506,19 @@ suite('SnippetController', () => {
 		}, ['this._', 'abc', 'def._']);
 
 	});
+
+	test('Multiple cursor and overwriteBefore/After, #16277', () => {
+		snippetTest((editor, cursor, codeSnippet, controller) => {
+
+			editor.setSelections([
+				new Selection(1, 5, 1, 5),
+				new Selection(2, 5, 2, 5),
+			]);
+
+			codeSnippet = CodeSnippet.fromTextmate('document');
+			controller.run(codeSnippet, 3, 0);
+			assert.equal(editor.getModel().getValue(), '{document}\n{document && true}');
+
+		}, ['{foo}', '{foo && true}']);
+	});
 });