Regression dragging blocks from the flyout

Tracking @microbit-robert's comment here https://github.com/RaspberryPiFoundation/blockly/pull/9903#issuecomment-4510634636 as a new issue.

Introduced in that PR, keyboard-motivated initial positioning has a negative impact on the mouse drag positioning, moving the block away from the mouse pointer.

https://github.com/user-attachments/assets/c0f00e3c-2171-48d5-a360-2ed1bff427ae

