I have read a sentence online
Elle est tombée quand elle est descendue au rez-de-chaussée de son immeuble ce matin.
In this case, why is it that we didn't use l'imparfait ? Isn't it the people fall down while she is walking down?
There is no way of telling whether the fall occurred when the person had already reached the ground floor or during their coming down the stairs. We would tend to believe the latter possibility is the right one but this use of word (somewhat colloquial in reason of its imprecision) does not allow the listener or reader to conclude. There is a matter of imprecision in the verb « descendre »: we don't know whether we are talking about the action of coming down or its result.
(TLFi) A. [Implique une idée de mouvement de haut en bas] 1. [Le haut et le bas marquent des niveaux différents dans un obj., une construction, un lieu] Aller (vers le bas), venir (d'en haut). Descendre de la chaire; descendre dans l'arène; descendre au salon.
A perfectly unequivocal manner of putting that would be as the Following ;
The idea of simultaneity is unequivocal as "comme" can't be used with the "passé composé" and the "imparfait" is necessary here as the other tenses ("présent historique" and "plus que parfait" make no sense TLFi).
"Quand" would also been used to the same effect, but necessarily in combination with the "imparfait" also.
(TLFi) [[quand]] [Pour marquer la simultanéité avec le fait exprimé par le verbe de la princ.] Au moment où, dans le temps que. Synon. lorsque (lang. écrite).
The use of the "imparfait" would force the first interprétation, there would then be no doubt: the fall would have occurred with certainty while the person went down the stairs. This tense would have been correct too but the reading of the sentence, as just explained would have been different.
Here are two instances that aim at showing the difficulty.
Did the pain manifest itself while eating or at the conclusion of the action? We understand generally that the time is during the action. Here too, the "imparfait" is better.
It's not clear whether "le temps pour autre chose" is during Learning or after; only the context will tell.