But what would be the UI/UX of it ? It's quite visible in land fights when troops are trying to capture a building, how do you want to do it in naval warfare ? Where the garrison isn't visible on the map ?

Well, capturing a fortress was a major strategic objective in many ancient situations...

I am not sure is sdl, because shift+scroll works for Camera Rotation, that implies the command is being received by the Game. Even I updated libSDL2.dylib and nothing. I think is not defined, maybe if I map it to change batch size may fix it. I was trying to add hotkey.gui.session.batchtrainingsize.increment = "Shift+WheelUp" hotkey.gui.session.batchtrainingsize.decrement = "Shift+WheelDown"  to the .cfg file but I don’t know very well the syntax. I did a second Partition with Windows in my MacBook and there it works (i don’t stay in Windows because recording is not working properly.)  

We need to debug why SDL is not picking it up. My guess is that it sends a different key code we're not handling on macOS EDIT: Seems to be working in the SVN version, maybe a recent SDL update.

I need to add something, because in macOS is not working for me the wheelUp/wheelDown for increment/decrement batch size (but is working for zoom and for camera rotation) How can I add it?