So lets say I'm a Fighter (F), and a Creature (C) is standing to my side, within my reach (*), like this:
o o o o o
o * * * o
o * F * o
o * C * o
o o o o o
If that creature attempts to move past me (to the left or right) and exit my reach (into the o-zone), will the reaction attack from the Tunnel Fighter fighting style (from Unearthed Arcana: Light, Dark, Underdark!) trigger as he's leaving my reach? I would guess yes, because he has to move a few inches to begin the process of leaving my reach, at which point he's "moved more than 5 feet", which triggers Tunnel Fighter's reaction.
If I'm right about that, Tunnel Fighter and the Sentinel feat (PHB, p. 169-170) have an interesting interaction that I'd like clarification on. Assuming I'm in the Tunnel Fighter stance when this happens, I can potentially take an Attack of Opportunity upon the creature as he leaves my reach and use my Reaction to attack him with Tunnel Fighter's trigger. However, Sentinel makes that AO stop the creature in its tracks.
Can I take both an Opportunity Attack and the Tunnel Fighter reaction attack in this particular situation?
If the creature starts in the bottom-right *, I assume the answer is a cut and dry "Yes", since he's clearly "moving more than 5 feet while within my reach" and won't get stopped by Sentinel until he's already moved 10 feet. The uncertainty arises when Sentinel's OA effect prevents him from taking the second 5 feet of movement.
And in the same vein, it's clear that the answer is "No" if the creature moves directly down. It has left my reach before it has moved more than 5 feet, so Tunnel Fighter's reaction attack definitely won't trigger.