IMNSHO, this is not a good solution. My colleague opened a PR which contained more than thirty changes. Five were changes I was interested in and the others were new, large JSON files that were added to the repo. During the review, the files I need to see weren’t located contiguously, so I had to painstakingly… click… each… collapse… button… as… you… described… one… by… one. 😛
GH needs to have controls in PR code reviews for collapse all, expand all, and probably toggle all. There should also be an setting to start reviews with all changes collapsed.