fix: "Invariant Violation: Expected targetIds to be registered."

We've sporadically seen this error from customers. The call stack
indicates that it's originating from HTML5BackendImpl's
handleTopDragOver's requestAnimationFrame callback.  I've been unable to
reproduce it locally; however, if I simulate a slowdown by replacing
`requestAnimationFrame(callback)` with `setTimeout(callback, 10000)`, I
can fairly reliably reproduce this error.

To fix it, I believe HTML5BackendImpl should consistently clear the
hover animation whenever the drag operation is ended for any reason.

I locally tested this fix in the v15.1.2 tag, and it appeared to work.

Fixes #763, #3403

CONTRIBUTING.md

@@ -1,6 +1,6 @@
 # Contributing to React-DnD
 
-So you want to contibute to React-DnD? Thank you! This library is a community effort, and your contributions are greatly appreciated.
+So you want to contribute to React-DnD? Thank you! This library is a community effort, and your contributions are greatly appreciated.
 
 ## FAQ
 

packages/backend-html5/src/HTML5BackendImpl.ts

@@ -347,6 +347,7 @@ export class HTML5BackendImpl implements Backend {
 		if (this.clearCurrentDragSourceNode() && this.monitor.isDragging()) {
 			this.actions.endDrag()
 		}
+		this.cancelHover()
 	}
 
 	private setCurrentDragSourceNode(node: Element | null) {
@@ -400,6 +401,33 @@ export class HTML5BackendImpl implements Backend {
 		return false
 	}
 
+	private scheduleHover = (dragOverTargetIds: string[] | null) => {
+		if (
+			this.hoverRafId === null &&
+			typeof requestAnimationFrame !== 'undefined'
+		) {
+			this.hoverRafId = requestAnimationFrame(() => {
+				if (this.monitor.isDragging()) {
+					this.actions.hover(dragOverTargetIds || [], {
+						clientOffset: this.lastClientOffset,
+					})
+				}
+
+				this.hoverRafId = null
+			})
+		}
+	}
+
+	private cancelHover = () => {
+		if (
+			this.hoverRafId !== null &&
+			typeof cancelAnimationFrame !== 'undefined'
+		) {
+			cancelAnimationFrame(this.hoverRafId)
+			this.hoverRafId = null
+		}
+	}
+
 	public handleTopDragStartCapture = (): void => {
 		this.clearCurrentDragSourceNode()
 		this.dragStartSourceIds = []
@@ -429,6 +457,7 @@ export class HTML5BackendImpl implements Backend {
 		// Avoid crashing if we missed a drop event or our previous drag died
 		if (this.monitor.isDragging()) {
 			this.actions.endDrag()
+			this.cancelHover()
 		}
 
 		// Don't publish the source just yet (see why below)
@@ -530,6 +559,7 @@ export class HTML5BackendImpl implements Backend {
 			// Only proceed if we have not handled it already.
 			this.actions.endDrag()
 		}
+		this.cancelHover()
 	}
 
 	public handleTopDragEnterCapture = (e: DragEvent): void => {
@@ -622,20 +652,7 @@ export class HTML5BackendImpl implements Backend {
 		this.altKeyPressed = e.altKey
 		this.lastClientOffset = getEventClientOffset(e)
 
-		if (
-			this.hoverRafId === null &&
-			typeof requestAnimationFrame !== 'undefined'
-		) {
-			this.hoverRafId = requestAnimationFrame(() => {
-				if (this.monitor.isDragging()) {
-					this.actions.hover(dragOverTargetIds || [], {
-						clientOffset: this.lastClientOffset,
-					})
-				}
-
-				this.hoverRafId = null
-			})
-		}
+		this.scheduleHover(dragOverTargetIds)
 
 		const canDrop = (dragOverTargetIds || []).some((targetId) =>
 			this.monitor.canDropOnTarget(targetId),
@@ -672,6 +689,7 @@ export class HTML5BackendImpl implements Backend {
 		if (this.isDraggingNativeItem()) {
 			setTimeout(() => this.endDragNativeItem(), 0)
 		}
+		this.cancelHover()
 	}
 
 	public handleTopDropCapture = (e: DragEvent): void => {
@@ -709,6 +727,7 @@ export class HTML5BackendImpl implements Backend {
 		} else if (this.monitor.isDragging()) {
 			this.actions.endDrag()
 		}
+		this.cancelHover()
 	}
 
 	public handleSelectStart = (e: DragEvent): void => {