After reading What can pantheon shield block?, I found out that on-hit effects are still applied (such as Teemo's Toxic Shot) while the basic attack is blocked. This implies that the damage for such abilities is split into two parts: the damage of the basic attack itself and the damage from any on-hit effects. Pantheon's passive blocks the first part while the second part is unaffected.
As Teemo's on-hit effect is physical/magic damage split (physical for his basic attack and magic for his DoT), the concept is easy to picture. But, how does this apply to Gangplank's and Ezreal's Qs (Parrrley and Mystic Shot, respectively). Each of these abilities tack on additional physical damage, but is this considered an on-hit effect or simply a stronger basic attack? Additionally, how does this apply to items that add physical damage on-hit, such as Blade of the Ruined King?