The search field we have right now is of a fixed width. And especially on more narrow screens that is particularly annoying, since tabs quickly drop to the next line. On the other hand, if you have a 27" screen, you almost have to look for the search field with a literal magnifier to find it.
For the past months, we have had a CSS change live on the English Wikipedia, that changes this for browsers that support viewport units. It has had 0 negative feedback, since it was deployed. It's time to bring it to the rest of the users.