What?

Fixes: #52919

Fix issue with the List View's drag-and-drop where sometimes blocks would suddenly be sent down to the bottom of the document when clicking around quickly.

Why?

From some logging out of values, what appears to be happening on trunk is that if you click on a selected block very rapidly with a little bit of movement, sometimes an onDragEnd callback is fired (which sets target to null), before the onDrop callback is fired.

What's expected to happen is that dragging on a selected block will effectively result in no moving of blocks occurring because the target is the same as what's being dragged over. However, the code that determines the desired target is throttled to 200ms, so if you go to drag a block and within that 200ms perform another drag, it's possible to fire that second drag after the first dragEnd event has cleared out the target and before the onDragOver gives us a new target.

I'm not 100% that this is exactly what's happening, but what I've observed is that if you're clicking around very quickly on a selected block, it seems that you can accidentally drag a block and rapidly start another drag in the same position, and due to this issue of target being set to null for a short time, the block winds up being sent all the way to the bottom.

To resolve this issue (which seems to be something of a race condition to me), we can check that the target is truthy before allowing a drop to occur.

How?

Within the onDrop callback, check that the target is truthy before allowing onBlockDrop to be called. This should be pretty safe as the list view drag and drop behaviour expects that there is a target set for any kind of drop to occur.

Testing Instructions

1. Before applying this PR, check that you can reproduce the issue described in List view: Blocks jump the bottom of the list after some drag & drops #52919
2. With this PR applied, try rapidly clicking on a block in the list view — it shouldn't flick the block to the bottom of the list
3. Ensure that dragging to different levels of the block hierarchy (in and out of Group blocks, and directly below nested Group blocks) works as on trunk
4. Ensure that dragging an image file from your computer's desktop into the List View results in an Image block being created with that image (this just checks that our truthy check for target doesn't interrupt drag events that are not about rearranging existing blocks)

Screenshots or screencast

Before	After