src/vs/editor/common/viewModel/overviewZoneManager.ts/OverviewZoneManager/constructor
class OverviewZoneManager:	constructor(getVerticalOffsetForLine: (lineNumber: number) => number) {
		this._getVerticalOffsetForLine = getVerticalOffsetForLine;
		this._zones = [];
		this._colorZonesInvalid = false;
		this._lineHeight = 0;
		this._domWidth = 0;
		this._domHeight = 0;
		this._outerHeight = 0;
		this._pixelRatio = 1;

		this._lastAssignedId = 0;
		this._color2Id = Object.create(null);
		this._id2Color = [];
	}