_getNextMinZIndex(iCurrent) {
			// deduct indices reserved from current z-index
			const iMaxZIndex = iCurrent - Z_INDICES_RESERVED;
			// initial minimum z-index
			const iMinZIndex = iCurrent - Z_INDEX_STEP;
			const iNextZIndex = getLastZIndex(iMinZIndex, iMaxZIndex, _aAssignedZIndices);
			_aAssignedZIndices.push(iNextZIndex);
			return iNextZIndex;
		}
Returns the next minimum possible z-index based on the passed z-index value.
@param {int} iCurrent Current z-index value
@returns {int} The next minimum z-index value
@private