fix: allow dragging the element inside a scrollable container

BREAKING CHANGE: A cloned element is now created when the element is being dragged, and the dragged
element is set to be positioned fixed. This may break some apps in some edge cases.

Fixes #25

src/draggable.directive.ts

@@ -135,7 +135,7 @@ export class DraggableDirective implements OnInit, OnChanges, OnDestroy {
    * @hidden
    */
   constructor(
-    public element: ElementRef,
+    public element: ElementRef<HTMLElement>,
     private renderer: Renderer2,
     private draggableHelper: DraggableHelper,
     private zone: NgZone
@@ -206,19 +206,47 @@ export class DraggableDirective implements OnInit, OnChanges, OnDestroy {
             .pipe(takeUntil(merge(this.pointerUp, this.pointerDown)))
             .pipe(share());
 
-          pointerMove.pipe(take(1)).subscribe(() => {
+          const dragStart$ = pointerMove.pipe(take(1)).pipe(share());
+          const dragEnd$ = pointerMove.pipe(takeLast(1)).pipe(share());
+
+          dragStart$.subscribe(() => {
             pointerDownEvent.event.preventDefault();
 
             this.zone.run(() => {
               this.dragStart.next({ x: 0, y: 0 });
             });
 
+            if (this.ghostDragEnabled) {
+              const rect = this.element.nativeElement.getBoundingClientRect();
+              const clone = this.element.nativeElement.cloneNode(true);
+              this.renderer.setStyle(clone, 'visibility', 'hidden');
+              this.element.nativeElement.parentNode!.insertBefore(
+                clone,
+                this.element.nativeElement
+              );
+
+              this.setElementStyles({
+                position: 'fixed',
+                top: `${rect.top}px`,
+                left: `${rect.left}px`
+              });
+
+              dragEnd$.subscribe(() => {
+                clone.parentElement!.removeChild(clone);
+                this.setElementStyles({
+                  position: '',
+                  top: '',
+                  left: ''
+                });
+              });
+            }
+
             this.setCursor(this.dragCursor);
 
             this.draggableHelper.currentDrag.next(currentDrag);
           });
 
-          pointerMove.pipe(takeLast(1)).subscribe(({ x, y }) => {
+          dragEnd$.subscribe(({ x, y }) => {
             this.zone.run(() => {
               this.dragEnd.next({ x, y });
             });
@@ -272,7 +300,7 @@ export class DraggableDirective implements OnInit, OnChanges, OnDestroy {
   }
 
   ngOnChanges(changes: SimpleChanges): void {
-    if (changes['dragAxis']) {
+    if (changes.dragAxis) {
       this.checkEventListeners();
     }
   }
@@ -459,4 +487,10 @@ export class DraggableDirective implements OnInit, OnChanges, OnDestroy {
       delete (this as any).eventListenerSubscriptions[type];
     });
   }
+
+  private setElementStyles(styles: { [key: string]: string }) {
+    Object.keys(styles).forEach(key => {
+      this.renderer.setStyle(this.element.nativeElement, key, styles[key]);
+    });
+  }
 }

test/draggable.directive.spec.ts

@@ -564,4 +564,37 @@ describe('draggable directive', () => {
       fixture.componentInstance.dragEnd
     );
   });
+
+  it('should create a clone of the element and make the host static', () => {
+    const draggableElement: HTMLElement =
+      fixture.componentInstance.draggable.element.nativeElement;
+    expect(
+      (draggableElement.previousElementSibling as HTMLElement).hasAttribute(
+        'mwldraggable'
+      )
+    ).to.be.false;
+    triggerDomEvent('mousedown', draggableElement, { clientX: 5, clientY: 10 });
+    triggerDomEvent('mousemove', draggableElement, { clientX: 7, clientY: 10 });
+    expect(draggableElement.style.position).to.equal('fixed');
+    expect(draggableElement.style.top).to.be.ok;
+    expect(draggableElement.style.left).to.be.ok;
+    expect(draggableElement.previousSibling).to.be.ok;
+    expect(
+      (draggableElement.previousElementSibling as HTMLElement).style.visibility
+    ).to.equal('hidden');
+    expect(
+      (draggableElement.previousElementSibling as HTMLElement).hasAttribute(
+        'mwldraggable'
+      )
+    ).to.be.true;
+    triggerDomEvent('mouseup', draggableElement, { clientX: 7, clientY: 8 });
+    expect(draggableElement.style.position).not.to.be.ok;
+    expect(draggableElement.style.top).not.to.be.ok;
+    expect(draggableElement.style.left).not.to.be.ok;
+    expect(
+      (draggableElement.previousElementSibling as HTMLElement).hasAttribute(
+        'mwldraggable'
+      )
+    ).to.be.false;
+  });
 });