要使弹出窗口（覆盖层）可拖动，可以使用Angular5和OpenLayers5结合的方法。以下是一个示例代码：

HTML模板：

  

  

    
Popup Window
    
This is a draggable popup window.
    
X
  

CSS样式：

.map-container {
  position: relative;
  width: 500px;
  height: 400px;
}

.map {
  width: 100%;
  height: 100%;
}

.popup {
  position: absolute;
  z-index: 1000;
  background-color: #fff;
  border: 1px solid #ccc;
  padding: 10px;
}

.close {
  position: absolute;
  top: 5px;
  right: 5px;
  cursor: pointer;
}

组件代码：

import { Component, OnInit, ViewChild, ElementRef } from '@angular/core';
import { Map, View } from 'ol';
import { Overlay } from 'ol/Overlay';
import { fromLonLat } from 'ol/proj';

@Component({
  selector: 'app-map',
  templateUrl: './map.component.html',
  styleUrls: ['./map.component.css']
})
export class MapComponent implements OnInit {
  @ViewChild('mapDiv') mapDiv: ElementRef;
  
  map: Map;
  overlay: Overlay;
  popupVisible: boolean = false;
  popupPosition: { left: number, top: number } = { left: 0, top: 0 };
  dragStartPos: { x: number, y: number } = { x: 0, y: 0 };

  constructor() { }

  ngOnInit() {
    this.initMap();
  }

  initMap() {
    this.map = new Map({
      target: this.mapDiv.nativeElement,
      view: new View({
        center: fromLonLat([0, 0]),
        zoom: 2
      })
    });

    // 创建覆盖层
    this.overlay = new Overlay({
      element: document.querySelector('.popup'),
      positioning: 'center-center',
      stopEvent: false
    });

    this.map.addOverlay(this.overlay);
  }

  startDrag(event) {
    if (event.button === 0) {
      this.dragStartPos.x = event.clientX - this.overlay.getPosition()[0];
      this.dragStartPos.y = event.clientY - this.overlay.getPosition()[1];
      document.addEventListener('mousemove', this.doDrag.bind(this));
      document.addEventListener('mouseup', this.stopDrag.bind(this));
    }
  }

  doDrag(event) {
    this.popupPosition.left = event.clientX - this.dragStartPos.x;
    this.popupPosition.top = event.clientY - this.dragStartPos.y;
  }

  stopDrag() {
    document.removeEventListener('mousemove', this.doDrag);
    document.removeEventListener('mouseup', this.stopDrag);
  }

  openPopup() {
    this.popupVisible = true;
  }

  closePopup() {
    this.popupVisible = false;
  }

}

在组件的ngOnInit方法中，初始化地图和覆盖层。在startDrag方法中，记录鼠标点击时的位置，并注册mousemove和mouseup事件监听器。在doDrag方法中，根据鼠标移动的距离更新覆盖层的位置。在stopDrag方法中，移除事件监听器。openPopup和closePopup方法用于控制弹出窗口的显示和隐藏。

通过以上代码，可以实现在地图上使用OpenLayers5创建一个可拖动的弹出窗口。