GTK3 - textview transparency to show applications behind

Question

I am currently working on an existing Linux application called Xpad, which is a sticky notes application written in C. I try to implement transparency but I have a hard time achieving what I want. A sticky note looks like this.

I have a (toplevel) gtk_window with a gtk_textview. If the background color of the textview is set with the function gtk_widget_override_background_color() to a transparant color (GdkRGBA where alpha value is smaller than 1), the color of the gtk_window behind it, becomes more visible.

However, I don't want to see the gtk_window, but the applications behind the gtk_window, such as the browser, libreoffice, or the desktop.

If the gtk_window is set to a certain transparancy, either with gtk_widget_override_background_color() or with gtk_widget_set_opacity(), the whole widget, including the window decorations become (partially) transparent.

To make the relations between the visible objects more clear, I have created a diagram of the different parts, and marked the place where I believe the transparency issue is taking place.

Anybody have any ideas how to make the textview background transparent, without making the window decorations transparent, so I can see whatever is behind this application?

Answer 1

Set a proper RGBA visual for the widget

w = //some GtkWidget or GtkWidget derived klass (i.e. GtkWindow)
gtk_widget_set_app_paintable (w, TRUE); // important or you will get solid color

// the next 3 lines should be wrapped in a func which is also hooked to "screen-changed"
GdkScreen *screen = gtk_widget_get_screen (w);
GdkVisual *visual = gdk_screen_get_rgba_visual (screen);
gtk_widget_set_visual(w, visual);

gtk_widget_show_all(w);



g_signal_connect(G_OBJECT(w), "screen-changed", G_CALLBACK(screen_changed_contaniing_above_code), NULL);