I've had no problem with IME in a long while, but every single time I install a new machine I forget how to get fontconfig to behave "correctly" wrt monospace fonts when the locale is japanese. Somehow, as long as I have japanese fonts installed, it picks a non monospace font for monospace. Which makes e.g. <pre> sections in a browser look wrong, ghex display completely fucked up, etc.

Last time happens to have been two weeks ago, so now I know what I did this time: create a .config/fontconfig/fonts.conf file containing:

  <?xml version="1.0"?>
  <!DOCTYPE fontconfig SYSTEM "fonts.dtd">
  <fontconfig>
  <match target="pattern">
      <test qual="any" name="family">
          <string>monospace</string>
      </test>
      <edit name="family" mode="prepend" binding="strong">
          <string>Noto Mono</string>
      </edit>
  </match>
  </fontconfig>

(I used Noto Mono, but pick what you like) In case that's helpful to someone else...