@Urbanecm - thanks for bringing this up! A couple of notes/replies:

• We introduced fixed with to content pages to improve readability. You can read more about the rationale behind our decision and more information on the actual width we settled on our project page here: https://www.mediawiki.org/wiki/Reading/Web/Desktop_Improvements/Features/Limiting_content_width, as well as in our FAQ here: https://www.mediawiki.org/wiki/Reading/Web/Desktop_Improvements/Frequently_asked_questions#Why_is_the_width_of_the_content_limited?_Why_is_there_so_much_white_space?
• For special pages, which are meant to be scannable, we decided that a fixed width is not necessary, as the page is not meant to be read from top to bottom like an article. Thus, all special pages currently do not have a limited width.
• This does create some awkwardness in terms of the tabs placement and styling. We will be slowly improving this area over the course of the project. The first change we will make here is to move the search bar into the top of the page. This is already deployed on all of our early adopter wikis, as well as for all users that have opted in using their preferences. On our early adopter wikis, we are also running an A/B test to measure the increase (if any) in the amount of search sessions based on the new location. From the screenshot above, it seems that you have been bucketed into our control group (the likelihood of which is 50% on our early adopter wikis). The test will run for two weeks, after which, we expect all special pages to look like this:

I'm resolving this task for now, as all the behavior stated here is expected. That said, let us know if you have any questions, and we can continue the conversation here, on wiki, or on our general feedback task: T258277: A friendly place to share your initial reactions to the first release of desktop improvements.