Scribbles for my memory

Every character in kitty had a massive gap after it, like the cell width was double what it should be. The obvious first move — swap font_family in kitty.conf — did nothing. Neither did the version a second pair of hands tried. Two failed attempts, and the config looked completely normal.

The config wasn't the problem. monospace is just a fontconfig alias, and on this machine fontconfig was handing it to a CJK font:

$ fc-match monospace
NotoSansCJK-Regular.ttc: "Noto Sans Mono CJK SC" "Regular"

kitty bases its cell width on the primary font, and a CJK font's cell is full-width — so narrow ASCII glyphs end up stranded in the middle of wide blank cells. The same kitty.conf was perfectly fine on another machine, because over there fc-match monospace returns DejaVu Sans Mono.

What was poisoning the alias was a file Ubuntu's language-selector drops in:

$ grep -A6 '<family>monospace</family>' \
    ~/.config/fontconfig/conf.d/64-language-selector-prefer.conf
		<family>monospace</family>
		<prefer>
			<family>Noto Sans Mono CJK SC</family>
			<family>Noto Sans Mono CJK JP</family>
			...

It prepends the Noto CJK mono fonts to the monospace alias, so they outrank every Latin mono. Comment out that one <alias> block and monospace goes back to DejaVu:

$ fc-match monospace
DejaVuSansMono.ttf: "DejaVu Sans Mono" "Book"

The reload trap that made this take an hour

Here's the part that hurts. After fixing fontconfig, pressing ctrl+shift+F5 to reload kitty... still showed the gaps. It looked exactly like the fix hadn't worked — the same dead end that had already wasted two attempts.

kitty caches its fontconfig resolution in-process. ctrl+shift+F5 re-reads kitty.conf, but it never re-queried fontconfig, so a change to how monospace resolves never reached the running kitty. Only quitting and reopening kitty — a fresh process — picked it up.

So the rule, painfully earned: when a kitty font change "doesn't work," before you conclude the fix is wrong, fully restart kitty and re-check. The fix may have been right all along; you were just looking at a cached font. And when the symptom is uniformly wide gaps across every character, run fc-match monospace before touching kitty.conf at all.