border-color adds a new border in GtkScrolledWindow (GTK 3.18) ?

[Giovanni Panozzo <giovanni panozzo it>]

Hello to all. I have just subscribed to the ML.


I have a problem with GTK 3.18: I have an application with a 
GtkScrolledWindow, which contains a GtkViewport which contains a 
GtkDrawingArea.
Via CSS I set the background colors of GtkScrolledWindow and GtkDrawingArea.

Up to version 3.16 of GTK, all worked fine:




In GTK 3.18 (which means: current Arch and immediate future Ubuntu 
16.04), a strange border has appeared:
(here in green)




The border only appears when both conditions are true:

1. applying the background color with following CSS:
   #myscrolledwindow {  background-color: green; }
2. scrollbars are shown because the main window is smaller than the
   drawing area.


If you move the vertical scrollbar, a second green border on the left 
will appear, covering some important pixels of the drawing area.
If you move the horizontal scrollbar, a second green border will appear 
on the top, covering some important pixels of the drawing area.


Below you can find the full source code.

Is it a new bug or am I doing something wrong ?

Thank you
Giovanni

------------------------------------------------------------------------
Compile with
    gcc `pkg-config --cflags gtk+-3.0` -o gtk318problem gtk318problem.c 
`pkg-config --libs gtk+-3.0`

#include <gtk/gtk.h>


static gboolean
draw_callback (GtkWidget *widget, cairo_t *cr, gpointer data)
{
  guint width, height;
  GdkRGBA color;

  width = gtk_widget_get_allocated_width (widget);
  height = gtk_widget_get_allocated_height (widget);
  cairo_arc (cr,
             width / 2.0, height / 2.0,
             MIN (width, height) / 2.0,
             0, 2 * G_PI);

  gtk_style_context_get_color (gtk_widget_get_style_context (widget),
                               0,
                               &color);
  gdk_cairo_set_source_rgba (cr, &color);

  cairo_fill (cr);

 return FALSE;
}


static void
activate (GtkApplication* app,
          gpointer        user_data)
{
  GtkWidget *window;
  GtkWidget *sw;
  GtkWidget *vp;
  GtkWidget *da;

  GtkCssProvider  *provider;

  /* Apply CSS */
  provider = gtk_css_provider_new();

  gtk_css_provider_load_from_data (provider,
        "#myscrolledwindow {\n"
        "  xbackground-color: green;\n"
        "}\n"
        "#mydrawingarea {\n"
        "  background-color: #a0a0a0;\n"
        "}\n"
        , -1, NULL);

  gtk_style_context_add_provider_for_screen (gdk_screen_get_default(),
        GTK_STYLE_PROVIDER (provider),
        GTK_STYLE_PROVIDER_PRIORITY_APPLICATION);
  g_object_unref(provider);


  window = gtk_application_window_new (app);
  gtk_window_set_title (GTK_WINDOW (window), "Window");
  gtk_window_set_default_size (GTK_WINDOW (window), 200, 200);

  sw = gtk_scrolled_window_new(NULL, NULL);
  gtk_widget_set_name(sw, "myscrolledwindow");
  gtk_container_add (GTK_CONTAINER(window), sw);

  vp = gtk_viewport_new(NULL, NULL);
  gtk_container_add(GTK_CONTAINER(sw), vp);

  da = gtk_drawing_area_new();
  gtk_widget_set_name(da, "mydrawingarea");
  gtk_widget_set_size_request (da, 640, 480);
  gtk_container_add(GTK_CONTAINER(vp), da);

  g_signal_connect (G_OBJECT(da), "draw", G_CALLBACK (draw_callback), 
NULL);


  gtk_widget_show_all (window);
}

int main (int argc, char **argv)
{
  GtkApplication *app;
  int status;

  app = gtk_application_new ("org.gtk.example", G_APPLICATION_FLAGS_NONE);
  g_signal_connect (app, "activate", G_CALLBACK (activate), NULL);
  status = g_application_run (G_APPLICATION (app), argc, argv);
  g_object_unref (app);

  return status;
}

------------------------------------------------------------------------