When CSS on screen has effect?

[rossati]

Hello
I have set some CSS at screen level but they don't act immediately, but only at the second run (sometimes at the third).
This happens both in Julia command line and in Visual Studio.
Here below a minimal example:

using GTK3_jll
const libgtk = libgtk3
const libgdk = libgdk3
using Gtk, Gtk.ShortNames
function fg_set_CSS(css::String)
    provider = convert(GObject,ccall((:gtk_css_provider_new, libgtk), Ptr{GObject},()))
    display = convert(GObject,ccall((:gdk_display_get_default,libgdk), Ptr{GObject}, ()))
    screen = convert(GObject,ccall((:gdk_display_get_default_screen,libgdk),Ptr{GObject},(Ptr{GObject},),display))
    ccall((:gtk_style_context_add_provider_for_screen,libgtk),Cvoid,(Ptr{GObject},Ptr{GObject}),screen,provider)
    ccall((:gtk_css_provider_load_from_data,libgtk),Bool,(Ptr{GObject}, Ptr{UInt8}, Clong, Ptr{C_NULL}),
    provider, css,-1, Ptr{Ptr{Cchar}}())
    return provider
end
function fg_reset_CSS(provider)
  display = convert(GObject,ccall((:gdk_display_get_default,libgdk), Ptr{GObject}, ()))
  screen = convert(GObject,ccall((:gdk_display_get_default_screen,libgdk),Ptr{GObject},(Ptr{GObject},),display))
  ccall((:gtk_style_context_remove_provider_for_screen,libgtk),Cvoid,(Ptr{GObject},Ptr{GObject}),screen,provider)
end
CSSStyles = """
#fg_btn {background-image: image(#aaa);font: 20px \"sans\";color:red}
#fg_btn:hover {background-image: image(#eee)}
#fg_btn:active {background-image: image(#ddd)}
#fg_window {background: navy}
#sld {min-width: 180px;}
"""
pr = fg_set_CSS(CSSStyles)
g = GtkGrid()
fg_win = GtkWindow("Minimal example")
set_gtk_property!(fg_win, :name,"fg_window")
g[1,1] = GtkButton("\u22EF\u22EF")
set_gtk_property!(g[1,1], :name,"fg_btn")
sld = GtkScale(false, 0:30)
set_gtk_property!(sld, :value_pos, 1)
set_gtk_property!(sld, :name, "sld")
set_gtk_property!(sld, :digits, 3)
g[1,4] = sld
push!(fg_win,g)
#showall(fg_win)
@idle_add showall(fg_win)
signal_connect(fg_win, :destroy) do widget
  fg_reset_CSS(pr)   # remove screen styles
  Gtk.gtk_quit()
  println("See you later alligator")
end

Thanks for any suggestion
Rossati Giovanni