Hi bug-gnu-emacs,

Straight underlines get thicker when font size increases, but wavy underlines do not.  The problem is particularly visible on hi-resolution displays, where wavy underlines look really tiny. This problem can be observed on all displays by running the following and pressing C-x C-= a few times:

(font-lock-mode -1)
(insert (propertize "AAAAA" 'face '(:underline (:style line)))
        "\n" (propertize "AAAAA" 'face '(:underline (:style wave))))

I've seen the problem mentioned a few times online (https://emacs.stackexchange.com/questions/24713/change-size-of-wavy-underlines-for-hidpi-displays, https://www.reddit.com/r/emacs/comments/51wm7d/emacs_on_hidpi_displays/).

For xterm.c, the relevant function seems to be x_draw_underwave.

Thanks!
Clément.