src/vs/editor/common/viewModel/viewModelImpl.ts/ViewportStart/update
class ViewportStart:	public update(viewModel: IViewModel, startLineNumber: number): void {
		const position = viewModel.coordinatesConverter.convertViewPositionToModelPosition(new Position(startLineNumber, viewModel.getLineMinColumn(startLineNumber)));
		const viewportStartLineTrackedRange = viewModel.model._setTrackedRange(this._modelTrackedRange, new Range(position.lineNumber, position.column, position.lineNumber, position.column), TrackedRangeStickiness.NeverGrowsWhenTypingAtEdges);
		const viewportStartLineTop = viewModel.viewLayout.getVerticalOffsetForLineNumber(startLineNumber);
		const scrollTop = viewModel.viewLayout.getCurrentScrollTop();

		this._viewLineNumber = startLineNumber;
		this._isValid = true;
		this._modelTrackedRange = viewportStartLineTrackedRange;
		this._startLineDelta = scrollTop - viewportStartLineTop;
	}