it’s from Oxford practice grammar
A: Did you have a nice day with Tessa?
B: No, not really. When we …(we/drink) our coffee,she harried off home.
the book’s answer is had drunk and my answer is were drinking and i have no idea why it’s wrong.
She couldn’t go home at the same time as drinking the coffee, which is why the tense you used in incorrect.
‘She’ couldn’t, clearly but who are ‘we’?
‘We’ can be Tessa and B as is guessed from the context.
How can I use past continues in this sentence, in order to say in the middle of drinking the coffee she left the cafe and left her coffee unfinished?
She was drinking her coffee but she suddenly left the cafe without finishing it.
We were drinking our coffee when she suddenly left, leaving hers (her drink) unfinished.
She left in the middle of coffee.
I really appreciate. 
I think it will still be better to say (making the context clearer by itself): While we were drinking our coffee together, she suddenly left without finishing hers. (Also, it avoids repetition of the verb ‘leave’)
What I got from all of these answers is we need to show somehow something happened in the middle of the action, by using suddenly or others, if we don’t it’s past perfect then past simple which is the purpose of this question.
It is too long-winded to be a natural part of conversation.
That’s not a hard and fast rule. In this particular case it indicates the abruptness of the second action. Grammatically, the sentences don’t require ‘suddenly’. Semantically in this particular context, it improves the sentence.
Be wary of trying to find a ‘best fit’ rule for all contexts from a standalone example.