On desktop web, the page header switches from the logo to the icon paired with the topic’s name and categories. On mobile, it sticks with the header.
Wouldn’t mobile be the place you want the sticky topic title the most? You could hide the categories of it’s a space problem.
Expand the progress bar on mobile to see the topic title.
https://blog.discourse.org/wp-content/uploads/2017/01/d17-backscroll.mp4
Okay.
Why does Discourse have a sticky header on mobile?
There is no room for a topic title on mobile, as you yourself noted. Please tap on the header icons to learn what they do. Thanks!