Validate offset before passing it down

src/vs/editor/common/model.ts

@@ -1079,6 +1079,7 @@ export interface ITextBuffer {
 
 	getValueInRange(range: Range, eol: EndOfLinePreference): string;
 	getValueLengthInRange(range: Range, eol: EndOfLinePreference): number;
+	getLength(): number;
 	getLineCount(): number;
 	getLinesContent(): string[];
 	getLineContent(lineNumber: number): string;

src/vs/editor/common/model/linesTextBuffer/linesTextBuffer.ts

@@ -198,6 +198,10 @@ export class LinesTextBuffer implements ITextBuffer {
 		return this._lines.slice(0);
 	}
 
+	public getLength(): number {
+		return this._lineStarts.getTotalValue();
+	}
+
 	public getLineContent(lineNumber: number): string {
 		return this._lines[lineNumber - 1];
 	}

src/vs/editor/common/model/textModel.ts

@@ -630,8 +630,9 @@ export class TextModel extends Disposable implements model.ITextModel {
 		return this._buffer.getOffsetAt(position.lineNumber, position.column);
 	}
 
-	public getPositionAt(offset: number): Position {
+	public getPositionAt(rawOffset: number): Position {
 		this._assertNotDisposed();
+		let offset = (Math.min(this._buffer.getLength(), Math.max(0, rawOffset)));
 		return this._buffer.getPositionAt(offset);
 	}
 
@@ -890,7 +891,8 @@ export class TextModel extends Disposable implements model.ITextModel {
 
 	public modifyPosition(rawPosition: IPosition, offset: number): Position {
 		this._assertNotDisposed();
-		return this.getPositionAt(this.getOffsetAt(rawPosition) + offset);
+		let candidate = this.getOffsetAt(rawPosition) + offset;
+		return this.getPositionAt(Math.min(this._buffer.getLength(), Math.max(0, candidate)));
 	}
 
 	public getFullModelRange(): Range {