Touch: can't backpack code

https://scratch.mit.edu/discuss/topic/865743/

Expected Behavior

When dragging code to the backpack, it lights up.
When dropping code over the backpack, it gets stored in the backpack.

Actual Behavior

The backpack doesn't always light up.
When dropping the code normally, it doesn't get stored even if the backpack lights up. To get the code into the backpack, one has to activate the backpack with a different finger than the one the blocks are dragged with.

Steps to Reproduce

Steps to reproduce the behavior:

1. Go to the Scratch editor on a mobile while signed in.
2. Open the backpack.
3. Place a block in the sprite.
4. Drag and drop the block over the backpack
   → the block doesn't get stored

Screenshots

https://github.com/user-attachments/assets/3b24ca1d-24e0-408a-8757-633c97096547

System Details

Android, Chrome 138

まず、Chrome 138 を搭載した Android の Scratch エディターで backpack のドラッグ＆ドロップ動作を再現し、次にブロックのドラッグと backpack の有効化に関するタッチ処理を追跡します。1 本の指でブロックをドラッグすると backpack が常にハイライトされ、そこにドロップしたときにブロックが保存されれば完了です。