[cecilpl2]

Because most people like it and it improves metrics.

From the discussion on sticky header [0],

> Our preliminary results show that an overwhelming majority of test participants reported positive experience with a sticky header. Participants mentioned they enjoyed the ability to access important functionality from any part of the page.

> Overall, there was an average 15% decrease in scrolls per session by logged-in users on the 15 pilot wikis in the treatment group (with the new sticky header), compared to the control group (without the sticky header). This indicates that our hypothesis was correct - adding the sticky header to the page reduced the need to scroll to the top of the page significantly.

> there was a 2.8% and a 6.8% increase in the percent of people who were able to successfully complete at least one edit using the edit button within the sticky header,

[0] https://www.mediawiki.org/wiki/Reading/Web/Desktop_Improveme...

[coldpie]

It might improve stats for people who use the header, but I'm pretty sure that's a tiny fraction of users. I just want the screen space and consistent scrolling behavior, not a header popping in and out as you scroll around or reach the top of the page.