Hello Joseph,
Something that tricked me for a long time also. Many people don't realize F. has a 'mark current line' function, accessible from the View menu, or Ctrl-B by default. It's a useful feature for bookmarking lines of interest. You can have more than one mark. When you move off the line, it's highlighted in purple, just as you say. There is a 'clear all marks' function int he View menu as well.
Once you know about this, it's really useful, I find.
Andrew