لَمْ يَعْصِكَ طَرْفَةَ عَيْنٍ
“He had not disobeyed You for the blink of an eye.”
فَإِنَّ وَجْهَهُ لَمْ يَتَمَعَّرْ فِي سَاعَةٍ قَطُّ
“His face had never shown a change of displeasure at any time.”
طَرْفَةَ عَيْنٍ describes a very brief moment. لَمْ negates an action in the past when joined to a present-tense verb. يَتَمَعَّرْ refers to a change in facial expression or color; in context it conveys being troubled by wrongdoing.
The concern is not a prescribed facial expression by itself, but inward responsiveness and an appropriate ethical response.