> Most Chinese, Japanese or Korean characters are rendered twice as wide as most other characters, even in a monospace font.
Not just CJK characters, but also a lot of non-Latin characters and symbols (a canonical example being ↑). In the East Asian Width standard [1] they are classified as "ambiguous", which can be half-width or full-width depending on the user choice. (By the way thank you much for pointing this out, this is super non-obvious to non-CJK developers and consequently affects CJK developers!)
It greatly depends on both the font and user settings. As an example of the latter, iTerm2 has the relevant configuration in the session preferences: "Ambiguous characters are double-width".
Not just CJK characters, but also a lot of non-Latin characters and symbols (a canonical example being ↑). In the East Asian Width standard [1] they are classified as "ambiguous", which can be half-width or full-width depending on the user choice. (By the way thank you much for pointing this out, this is super non-obvious to non-CJK developers and consequently affects CJK developers!)
[1] https://unicode.org/reports/tr11/