fix validatePosition, fixes #15882

src/vs/editor/common/services/editorSimpleWorker.ts

@@ -212,19 +212,19 @@ class MirrorModel extends MirrorModel2 implements ICommonModel {
 			lineNumber = 1;
 			column = 1;
 			hasChanged = true;
-		}
-		else if (lineNumber >= this._lines.length) {
+
+		} else if (lineNumber > this._lines.length) {
 			lineNumber = this._lines.length;
 			column = this._lines[lineNumber - 1].length + 1;
 			hasChanged = true;
-		}
-		else {
+
+		} else {
 			let maxCharacter = this._lines[lineNumber - 1].length + 1;
 			if (column < 1) {
 				column = 1;
 				hasChanged = true;
 			}
-			else if (column >= maxCharacter) {
+			else if (column > maxCharacter) {
 				column = maxCharacter;
 				hasChanged = true;
 			}

src/vs/editor/test/common/services/editorSimpleWorker.test.ts

@@ -87,6 +87,11 @@ suite('EditorSimpleWorker', () => {
 		assertPositionAt(Number.MAX_VALUE, 4, 30);
 	});
 
+	test('ICommonModel#validatePosition, issue #15882', function () {
+		let model = worker.addModel(['{"id": "0001","type": "donut","name": "Cake","image":{"url": "images/0001.jpg","width": 200,"height": 200},"thumbnail":{"url": "images/thumbnails/0001.jpg","width": 32,"height": 32}}']);
+		assert.equal(model.offsetAt({ lineNumber: 1, column: 2 }), 1);
+	});
+
 	test('MoreMinimal', function () {
 
 		return worker.computeMoreMinimalEdits(model.uri.toString(), [{ text: 'This is line One', range: new Range(1, 1, 1, 17) }], []).then(edits => {