Drop-down menus should use common fate with a consistent flow of movement. 

Example 1

This image shows both the way to use common fate in drop-down menus and process illustration. 

Example 2

Common fate can be used for designing drop-down menus or illustrating a process.  You should make sure that elements that belong together move in the same direction with the same speed.

University of Michigan - Ann Arbor

The common fate principle states that "elements moving towards the same direction are perceived as more related than those moving in different directions, or not moving at all."

The Law of Common Fate

"What are Gestalt Principles?" Interaction Design. https://www.interaction-design.org/literature/topics/gestalt-principles

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