On Windows, no font is selected for fullwidth latin characters.

[xorgy]

Fullwidth latin characters are most commonly used in Japanese text, and are primarily supported in Japanese fonts, but such a font is not selected on Windows when displaying fullwidth latin characters in any context. For example, we try to render ‘ａ’ U+FF41 FULLWIDTH LATIN SMALL LETTER A

This issue has been encountered particularly when using input methods:

[crlf0710]

I'd say using only the Unicode scripts is simply the wrong approach here. First it's possible to determine that it belongs to East Asian scripts with data from https://www.unicode.org/reports/tr11/. Then among the East Asian languages (Japanese, Korean, Simplified Chinese, Traditional Chinese) using the shared pool of Unicode code points, the program should use some way (language metadata from text, system locale, or maybe simply user configuration) to choose the variant to use. Then it needs to pick up a font corresponding to that variant.