When I make a character wider by changing the char width table, it isn't
sometimes with shows and sometimes it doesn't.

1. Start up emacs with this command.

emacs -Q --eval '(progn (set-char-table-range char-width-table #x221e 2)
(set-char-table-range char-width-table #x2014 2))'

2. Type "—a" into the scratch buffer. The em dash should be correctly
rendered two cells wide, and the a shouldn't overlap with the em dash.
3. `M-x redraw-screen`. The em dash should now overlap the a.

consistently works with:

- Termux, iosevka
- xfce4-terminal, iosevka and source code pro
- Kitty, iosevka