Closes #4446

Before: If the unread article is <40 pixels below the top edge, it was marked as read
After: If the unread article is > 5 pixels above the top edge, it is marked as read

Changes proposed in this pull request:

• +40 pixels -> -5 pixels
• use a const for better documentation/readability of the source code (maybe you can find a better const name)

How to test the feature manually:

1. enable "Mark an article as read… while scrolling" (Config -> Reading)
2. enable "Hide articles after reading" (Config -> Reading)
3. have a list of unread articles
4. scroll down
5. wait
6. it does not autom. mark articles as read one by one

Pull request checklist:

• clear commit messages
• code manually tested