Here is the situation I am wondering about:
My Rogue is in melee with an opponent. On his turn he disengages, and moves his full movement away from his opponent. The opponent, on their turn, uses their full movement to put themselves within 5 feet of my Rogue again, ending their turn. Can my Rogue then use the Skirmisher ability and move an additional 1/2 speed, or is the reaction essentially wasted because my Rogue has already used his full movement this turn?
I am leaning towards no.
Best Answer
Yes, he can.
The Scout rogue's Skirmisher feature says:
Note that this ability uses your reaction, not your "movement". You don't have a pool of movement outside your turn, normally:
There are no general rules for willingly moving (as opposed to being forcefully moved by someone or something else) outside your turn. You normally have a pool of movement on your turn equal to your speed, and none outside it.
The Scout rogue's Skirmisher feature gives them a special way to move outside their turn. It lets them use their reaction when an enemy ends its turn within 5 feet of them, and specifies that the Scout can move a distance up to half its speed. This doesn't depend at all on how much (or how little) you moved on your turn. You can use this reaction if the requirement is met, regardless of whether you moved on your own turn.