The only reason I can think of is that some people like to have the hold key action repeatedly type the specified character and the compose menu gets in the way of that. This is a relatively clever way of keeping both features active without having to repeatedly resort to terminal commands.
Why is macosx system is better? Compose keys are mnemonic: e + . = ė ‒ you don't need to remember any abitrary codes and there are no popups or any implicit accidents.
I've tried many different schemes etc and compose key is still my favorite system and having unified system for linux and macos is big bonus for cross platform users. At the end of the day it's just another option! :)
No, it does not. Holding u briefly overlays a keyboard of "compositions" for u, and you can just press whatever key you want afterwards to pick one, see: https://osxdaily.com/2017/03/22/type-accents-mac-easy/
> Why is macosx system is better? Compose keys are mnemonic
Because macosx system is not mnemonic. If I need to type a particular accent, which I do very often, and there are O(50) variations of them, I don't have to remember anything. Instead, the computer shows me all the options as a type, and I just pick the one I want.
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If I need to type "u" 42 times for whatever reason, holding u and waiting sucks anyways. Every editor I use has a better more precise way of doing this (on my emacs its just meta+42+u), and worst case I can always just do cmd+space and execute "repeat 42 u" or similar.
On MacOSX, I just hold u, and a menu appears, that allows me to select ü by just pressing 2. So I end up just doing u+2. With the huge advantage that I don't have to remember the combinations.
Similar for emojis and pretty much anything else you'd like to type as others have mentioned.
So the only question i have is: why? Really. Why would anyone go through the trouble of porting compose keys to macosx, when macosx native solution is so much better?
Time would be better spent on porting MacosX solution to Linux, Windows, et. al.