Anchor htop to one specific process?

You can't anchor a given process line to the top but htop can follow a process if you press Shift+F. From the man page:

   F    "Follow" process: if the sort order causes the currently  selected
        process  to  move  in  the list, make the selection bar follow it.
        This is useful for monitoring a process: this way, you can keep  a
        process  always  visible  on  screen. When a movement key is used,
        "follow" loses effect.

Here I'm following my Firefox process:

Note that if you're only interested in one particular process, the -p <pid> will do what you want (12352 is the pid of Firefox on my system):

$ htop -p 12352