Not knowing what the next scroll will bring runs the same “variable reward” mechanism used in slot machines. The brain stays engaged by the possibility that the next scroll might bring something interesting.
That's why the feeling of “just a bit more” isn't about willpower, it's about design. The same mechanism quietly turns fifteen minutes into an hour without you noticing.
What actually helps isn't a strict rule, it's having something concrete for your hand to do when the urge hits, a short walk, putting on a sound, switching to a different tab.