Air Carrier Captain reported they initiated a go around at 100 feet due to a departing aircraft still on the runway.

Narrative

We were cleared for the Visual Approach into runway XXR in VMC conditions with about a 7 knot tailwind flying down final. As we approached runway XXR we were following an aircraft a few miles ahead of us going to the same runway. Runway XXL was closed at the time so ZZZ was using runway XXR for both takeoffs and landings at the time. When we reached about a two mile final tower cleared Aircraft X into position and hold on runway XXR. I told the first officer to prepare for a go around if needed. Just passing through 500' on final tower gave Aircraft X clearance to takeoff. When we reached 100' above the runway I commanded a go around as Aircraft X hadn't even started to rotate on the runway. During the go around I instructed the First Officer to turn right immediately as we were catching Aircraft X taking off. Called tower and told them we were going around and turning right. Tower gave a right turn to 320 and climb to 3000'. After leveling off and running through checklists; I informed the crew and passengers about the go around and prepared for a return to land. Executed a normal landing on runway XXR and parked at the gate.