I have seen this behavior in other text editors as well. Luckily, it's a presentational problem as the encoding is fine both when when pasting to and copying from the editor in my experience.
Funny enough, there are also editors where it looks fine; both TextMate and Textastic on Mac display the diacritics correctly in combination with e.g. IPA characters. Whether fallback fonts will display those combinations depends on what glyphs are available for the font you currently, use of course. Currently I've only had complete success with Menlo and Courier New in that area.
I find it too cumbersome to read in ST so I'm forced to use TextMate for any task that involves diacritics with special characters.
I would *love* to see this fixed as I work with IPA characters with diacritics on a daily basis (an entire dictionary for a language with no official writing system among other things) but I have no idea whether this is something that can be patched or would take a complete rewrite and/or change of text engine.
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