Some umlauts and extended Latin characters are shown as fullwidth

Question

In Vim, I find each of ü and ß takes up fullwidth, i.e. twice as fat as ASCII characters. I am writing some German-learning notes, and find this to be very confusing, since I have been mistaken that there were a whitespace following that character. How can I make them shown as halfwidth?

Afterwards, out of curiosity, I tried some extended Latin characters (umlauts, accented vowels, currency symbols, etc.), and find some other characters looking like halfwidth but be shown as fullwidth. They include (but I suppose are not restricted to)

ßüŒœø€§¶

This is true for Vim inside iTerm2, MacVim inside iTerm2, Vim inside Terminal.app, MacVim inside Terminal.app, and even for standalone GUI of MacVim. I have tried several monospace fonts, including Monaco and Menlo. I use Mac Sierra.

Answer 1

Sorry, it's because I set :set ambiwidth=double. It makes some characters twice as wide as usual. I set :set ambiwidth=single, and all is fine. This line, I must have copied from some stranger's vimrc and did not examine closely. It's my fault, not Vim's fault.

I have no idea what ambiwidth is for. Maybe there is some font that need it to display right (see :help 'ambiwidth'). But as I cancelled it and set to single, everything still seems fine, Chinese or English. This question is kept for future reader's convenience.