When middle clicking an item in the creative inventory into a slot, it should create a stack of 64 of the item that is being held by the cursor, but this no longer works in 1.17 unless the middle click mouse is dragged over multiple slots.

Steps to reproduce:

1. Go into creative mode
2. Give yourself one item that stacks to 64. Ex: /give @p diorite
3. Left click so that the one item is being held by the cursor.
4. Positioning the mouse over a single slot, middle click without moving the cursor.

The item should fall from the cursor into the slot, and go from one item to 64 items, but this will not happen in 1.17. This would be the case in previous versions (1.16.5).

Holding down middle mouse and dragging over several slots, however, does seem to work.