You are asking for a definitive answer when there simply isn't one. Different speakers will view the same situation in different ways. The same speaker may view identical situations in different ways depending on other things in his/her mind in the seconds leading up to the utterance.
When/While she read/was reading the newspaper, I fixed/was fixing my breakfast.
There are eight possible utterances there, not counting the possibilities with alternatives for read : peruse, study, scrutinise, glance over/through, etc.
Unless we are actually there, we have no idea what may be a 'more' or 'less' likely utterance.
Incidentally, "According to the rules of prescriptive grammar, while must occur before the past progressive and when before the simple past. Native speakers,however,will commonly substitute when for while before the past progressive."
It seems to me that the rules of prescriptive grammar referred to in your book are pointless if native speakers commonly ignore them.
Student or Learner