The affix ghañ comes after the verb stṝ 'to cover' when the preposition vi is in composition with it when the sense is that of extension and when such spreading does not refer to words.,
The word प्रथनं means 'extent, circuit, range'. As पटस्य विस्तारः 'the extension of the garment'.
Why do we say 'when meaning extent'? Observe, तृणविस्तरः 'a collection of grass'. Why do we say 'not referring to words'? Observe विस्तरो वचसाम् 'prolixity of words'.,