What is the word or idiom for a person who pays attention to small and unimportant things?

We were reading a text in my English class and One of my students Lost the line. I pointed to the line we were reading and instead of following the text she said teacher you have beautiful nails. Now I am wondering if there’s an adjective, idiom or so which shows disapproval for

a person who pays attention to small and unimportant things ?

Solution 1:

We could say that she -

cannot see the forest for the trees

fail to grasp the main issue because of over-attention to details.

OR

has gone off on a tangent

to pursue a somewhat related or irrelevant course while neglecting the main subject.

^{Source- http://idioms.thefreedictionary.com/go+off+on+a+tangent}

Hope this helps!

Solution 2:

One could be pedantic

One could be nitpicky.

One could be splitting hairs.

They have generally negative connotations.

Is it's a good thing then:

One could be paying attention to details.

One could be thorough.

One could be attentive.