The problem is also exists in Gedit, which is by default themed with Ambiance theme from `light-themes` package.
Problem confirmed on Ubuntu session (means GNOME) of Ubuntu 18.04.2 LTS and in upcoming 19.04 while running Unity desktop. Here only Adwaita theme looks correct.
So all modern GTK themes should be fixed to correctly show drop-down menu in GtkSourceView.
The problem is also exists in Gedit, which is by default themed with Ambiance theme from `light-themes` package.
Problem confirmed on Ubuntu session (means GNOME) of Ubuntu 18.04.2 LTS and in upcoming 19.04 while running Unity desktop. Here only Adwaita theme looks correct.
So all modern GTK themes should be fixed to correctly show drop-down menu in GtkSourceView.