src/vs/editor/common/cursor/cursor.ts/CompositionState/_capture
class CompositionState:	private static _capture(textModel: ITextModel, selections: Selection[]): CompositionLineState[] | null {
		const result: CompositionLineState[] = [];
		for (const selection of selections) {
			if (selection.startLineNumber !== selection.endLineNumber) {
				return null;
			}
			result.push(new CompositionLineState(
				textModel.getLineContent(selection.startLineNumber),
				selection.startColumn - 1,
				selection.endColumn - 1
			));
		}
		return result;
	}