On Mac keyboards, the bottom left key is the FUNCTION key but on Windows it is CONTROL. By default, Minecraft sets the default sprint key to CONTROL, but I always change it to FUNCTION as it is easier on a Mac keyboard.

In the latest snapshot (18w07a/b/c), I noticed a change in how Minecraft will save the set keybind for a control (options.txt). It no longer uses integers. For some reason, I can't set any key to FUNCTION anymore. It seems like Minecraft doesn't know what it is. There don't seem to be any errors in the console either. Please fix this ASAP as I always use it like this!