For Steam users, I found a fix at the platform level using
this third-party "Say No to Notch" app. I just installed it and haven't played around with it much yet so I can't 100% vouch for it, but Stardew's title screen looks good to me now.
The app lets you toggle the camera "notch" on/off by reducing your mac's total screensize. Someone else recommended an alternative (
BetterDisplay) as well, in case Say No to Notch breaks, but I haven't tried it. (
source)
If you got your Stardew from GOG as a
.app
file then
Lew Zealand's solution should work for you, so try that first. (My Terraria app had the same issue and I fixed it like this, for example.)
But for Steam's Stardew, other indie games, or unpackaged code that opens fullscreen windows, "Say No to Notch" seems like a useful tool to have around.