It is a bug, I have reported it to launchpad to notify the developers about the problem.
As temporary solution you can switch icon theme using GNOME Tweaks → Appearance to one of the following icon sets:
to get normal close button appearance:
Seems to be a real bug of Snap and theming of such applications on GNOME DE.
I got the same issue using fresh Ubuntu 19.10 installation on VirtualBox with GNOME FlashBack and Adwaita GTK and icon themes.
I have tested all themes (full list from
ls /usr/share/themes/ -1) and all icons (full list of
ls /usr/share/icons/ -1) which were listed in GNOME Tweaks → Appearance tab and can conclude that many icon sets are broken:
The icon set from above list render garbage on the close button.
The list of correct icon sets is very short:
You can test themes and icon sets on your machine with script below:
for t in $(ls -1 /usr/share/themes/)
for i in $(ls -1 /usr/share/icons/)
echo -e "\nTheme: $t, Icons: $i\n"
gsettings set org.gnome.desktop.interface gtk-theme "$t"
gsettings set org.gnome.desktop.interface icon-theme "$i"
#read -r -s -p "Press <Enter> for next theme"
Or use GNOME Tweaks to switch themes and icon sets.