I was trying to search for a word containing an umlaut, but I wasn't able to ender it after pressing /. After that I tried with other non-ascii characters and mupdf seems to ignore all of them. Regards
Incomplete patch for mupdf-x11 1.15 attached (works with utf-8 locales; there are display problems in some non-utf-8 locales - it is possible to input [ascii and non-ascii] symbols, but search string and prompt is invisible); XKB (tested with cyrillic keyboard layout) and IM input seems works (tested with LANG=ja_JP.UTF-8 and uim/anthy).