So, I've come across this sentence
which I'm fairly sure it means something like
At that time, something's shadow quickly crossed the sunlit lithograph.
Now, what surprised me was the word サッと. I know that カタカナ is sometimes used to emphasise the word (much like an italic or bold font). What I didn't know is that there are words written in a mix of ひらがな and カタカナ.
A quick search revealed this answer. From what I could surmise, three groups of words were mentioned:
When the final word is made of two other words (e.g. あんパン is made from joining あん and パン)
Foreign words made into verbs (e.g. ググる from ググル, ダブる from ダブル), in which a final る is written in ひらがな
The same concept as 2. but for adjectives (e.g. セクしい from セクシー)
Now, サッと doesn't match neither group. From the context of the sentence, I've connected it with 颯っと which is usually written in kana alone, according to Jisho.
In summary, why the mix of ひらがな and カタカナ in this case?