Hello Aldebaran and welcome to the community! It's great you and your friends like to play multiplayer. To let you get the best help, would you please post screenshot of the error message? Maybe it's something about special characters like ´`' …

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 imagine is not mapped in the intel version, (but with trackpad it works =S) 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 made a second Partition with Windows in my MacBook and there it works (I have been not able to configure well, my monitor is 4K so when I use smaller resolution I get a giant cursor xD.)  

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.