Dragging a stack behind the palette can delete part of it

[cwillisf]

While dragging a block, you can connect it to blocks that sit behind the block palette. When you release, it connects and deletes all blocks below the connection point — silent code loss.

• Repro video (@redspacecat): https://github.com/user-attachments/assets/23ff8511-103d-4ec2-abe3-d9c4fea4e8fc
• Forum: https://scratch.mit.edu/discuss/post/9112451/

Reported by @redspacecat; repro confirmed by @kaylen-s.