src/vs/editor/browser/viewParts/overviewRuler/overviewRuler.ts/OverviewRuler/constructor
class OverviewRuler:	constructor(context: ViewContext, cssClassName: string) {
		super();
		this._context = context;
		const options = this._context.configuration.options;

		this._domNode = createFastDomNode(document.createElement('canvas'));
		this._domNode.setClassName(cssClassName);
		this._domNode.setPosition('absolute');
		this._domNode.setLayerHinting(true);
		this._domNode.setContain('strict');

		this._zoneManager = new OverviewZoneManager((lineNumber: number) => this._context.viewLayout.getVerticalOffsetForLineNumber(lineNumber));
		this._zoneManager.setDOMWidth(0);
		this._zoneManager.setDOMHeight(0);
		this._zoneManager.setOuterHeight(this._context.viewLayout.getScrollHeight());
		this._zoneManager.setLineHeight(options.get(EditorOption.lineHeight));

		this._zoneManager.setPixelRatio(options.get(EditorOption.pixelRatio));

		this._context.addEventHandler(this);
	}