The Forest Terrain clarifications say:

forest: (terrain) Forest squares are considered difficult terrain. Forest provides cover against ranged attacks. A creature in a forest square has cover against melee attacks unless the attacker is adjacent to a nonforest square in the creature's space. (Reminder: Melee Reach attacks count as ranged attacks for purposes of all cover rules, including these.) Line of sight (but not line of effect) is blocked if the line touches a corner of a forest square or 2 edges of the same forest square. A creature can still trace line of sight to creatures and squares in its own space as well as to adjacent creatures and squares.
Additional Forest clarifications:
Blindsight does not allow a creature to see through forest. The Blindsight glossary definition lists the things that it counters, and neither Forest terrain nor the effects of Forest terrain are among those things.
Against ranged attacks and Melee Reach attacks, a large or larger creature always gets cover from forest terrain when it's in at least one square of forest terrain.
Against non-Melee Reach melee attacks, a large or larger creature always gets cover when it is entirely in forest.
However, a large or larger creature is only partially in forest gets cover only if the attacker isn't adjacent to a nonforest square in the creature's space.
If forest gives a creature cover versus all adjacent enemies' melee attacks, that creature can move without provoking AoOs, can make ranged attacks, and can cast non-touch spells even if it is threatened by those enemies.


Dave