I have found this question online and realized that "ice" should actually be "eyes", which makes more sense with the context of the sentence.
Answer:
The entire absolute phrase in the sentence is "eyes glittering with concentration".
Explanation:
After reading it, we realize that "eyes glittering with concentration" refers to the sentence as whole. That is what makes it an absolute phrase - instead of modifying just one noun, it modifies a whole sentence. In this case, we can imagine Jack moving his cursor while his eyes glitter, showing how focused he is.
Absolute phrases' structure include a noun or a pronoun followed by a participial phrase. For example: hands trembling with nervousness. Notice that "eyes glittering with concentration" follows that structure.
Participial phrases consist of a participle and modifiers. For example: running out of energy.
