src/vs/editor/common/viewModel/viewModelImpl.ts/ViewModel/getCompletelyVisibleViewRange
class ViewModel:	public getCompletelyVisibleViewRange(): Range {
		const partialData = this.viewLayout.getLinesViewportData();
		const startViewLineNumber = partialData.completelyVisibleStartLineNumber;
		const endViewLineNumber = partialData.completelyVisibleEndLineNumber;

		return new Range(
			startViewLineNumber, this.getLineMinColumn(startViewLineNumber),
			endViewLineNumber, this.getLineMaxColumn(endViewLineNumber)
		);
	}