Will Booming Blade trigger before the Sentinel feature takes effect?

@chopkins779Will booming blade trigger before the Sentinel feature takes effect? Opportunity attacks happen just before they leave reach. i believe the creature then takes damage, unless it opts to stop moving immediately and stay in your reach

— Mike Mearls (@mikemearls) January 30, 2016

@chopkins779 When is booming blade’s effect triggered? Does the creature have to expend a full 5 feet of movement? essentially, yes. that's enough to move from one spot to another for game purposes

— Mike Mearls (@mikemearls) January 30, 2016