How to select a part of a geometry?

I want to be able to highlight parts of a geometry, example highlight 1/3 of one of the box’s faces.

I looked at decals because of this threejs example, are there any other approaches?

You could generate a new geometry from the selection and then create a new mesh with a special material for highlighting.

Did you see that
( examples 2019)
from the collection
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Maybe this approach will be also helpful: How to get the point index in the path of TubeBufferGeometry when moving mouse on the TubeBufferGeometry?