Inspired by this question, I was curious if removing one vertical column of tiles would change the result. Id est for the following configuration:
H░█░
░█░M
is the Monster (M) a valid target for a ranged Hero (H) attack through the Obstacles (█)?
My thoughts are that M is a valid target for H because a line can traced from the top right corner of H to the bottom left corner of M that merely passes through the corner (specifically not along the edge) of both obstacles.
TL;DR Yes
Please have a look at the below given sketch. The line through the upper right corner A = (1,2) of the ranged Hero and the bottom left corner B = (3,0) of the monster is described by the equation y = -x+3. This line doesn't intersect any of both given obstacles since for any x < 2 the corresponding value of y is greater than 1 and for any x > 2 is smaller than 1. Hence, the solely common point is C = (2,1). Since the line (quoting "Line of Sight" (page 12) from the Rules of Play)
only touches the corner of a blocked space (without passing through the space itself ), the target space is in line of sight.
