A new world record! A stack of 222 Drengin fighters, none of which have been combined into a fleet.
I would also like to reinforce a post I saw a while ago from someone else (I do not remember who). The issue revolves around starting the invasion of a planet when the invasion is not going to be immediately successful. If you look at the screen snapshot below as an example, Ebla II has 2 fighters defending the planet and a stack of 222 fighters in the cell adjacent to Ebla II. I can attack the planet with a reasonably strong ship to take out the 2 fighters and then start the invasion with another fleet. On the Drengin's next turn they will take 2 of the fighters from the stack of 222 and put them on Ebla II which ends my invasion. This cycle will continue for 111 turns unless my invading fleet can complete the invasion immediately.
I strongly recommend that you change the logic so that, if a planet is currently being invaded, ships from the defending civilization must defeat the invading fleet before they can land on the planet. Allowing a weak fighter to end an invasion simply by landing on the planet is not a good mechanic.