Initial wxTLW::(Can)SetTransparent support for wxGTK. The return value of SetTransparent is currently very

optimistic and doesn't use the EWMH composite manager spec update proposal (_NET_WM_CM_Sn) yet. This is on purpose
until the proposal appears in EWMH new version draft and it gets used by more composite managers.
See http://www.nabble.com/Interaction-between-applications-and-compositing-managers-t1389248.html and in the
future http://standards.freedesktop.org/wm-spec/latest/ for details.
Some code reordering is probably in order (helper function to utilsx11?) and testing on non-traditional X servers
appreciated.


git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@40192 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/gtk/toplevel.h

@@ -69,6 +69,9 @@ public:
     virtual void SetTitle( const wxString &title );
     virtual wxString GetTitle() const { return m_title; }
 
+    virtual bool SetTransparent(wxByte alpha);
+    virtual bool CanSetTransparent();
+
     // Experimental, to allow help windows to be
     // viewable from within modal dialogs
     virtual void AddGrab();

src/gtk/toplevel.cpp

@@ -1300,3 +1300,57 @@ void wxTopLevelWindowGTK::SetWindowStyleFlag( long style )
     }
 #endif // GTK+ 2.2
 }
+
+#include <X11/Xlib.h>
+
+/* Get the X Window between child and the root window.
+   This should usually be the WM managed XID */
+static Window wxGetTopmostWindowX11(Display *dpy, Window child)
+{
+    Window root, parent;
+    Window* children;
+    unsigned int nchildren;
+
+    XQueryTree(dpy, child, &root, &parent, &children, &nchildren);
+    XFree(children);
+
+    while (parent != root) {
+        child = parent;
+        XQueryTree(dpy, child, &root, &parent, &children, &nchildren);
+        XFree(children);
+    }
+
+    return child;
+}
+
+bool wxTopLevelWindowGTK::SetTransparent(wxByte alpha)
+{
+    if (!m_widget || !m_widget->window || !CanSetTransparent())
+        return false;
+
+    Display* dpy = GDK_WINDOW_XDISPLAY (m_widget->window);
+    // We need to get the X Window that has the root window as the immediate parent
+    // and m_widget->window as a child. This should be the X Window that the WM manages and
+    // from which the opacity property is checked from.
+    Window win = wxGetTopmostWindowX11(dpy, GDK_WINDOW_XID (m_widget->window));
+
+    unsigned int opacity = alpha * 0x1010101;
+
+    // Using pure Xlib to not have a GTK version check mess due to gtk2.0 not having GdkDisplay
+    if (alpha == 0xff)
+        XDeleteProperty(dpy, win, XInternAtom(dpy, "_NET_WM_WINDOW_OPACITY", False));
+    else
+        XChangeProperty(dpy, win, XInternAtom(dpy, "_NET_WM_WINDOW_OPACITY", False),
+                        XA_CARDINAL, 32, PropModeReplace,
+                        (unsigned char *) &opacity, 1L);
+    XSync(dpy, False);
+    return true;
+}
+
+bool wxTopLevelWindowGTK::CanSetTransparent()
+{
+    int opcode, event, error;
+    // Check for the existence of a RGBA visual instead?
+    return XQueryExtension(gdk_x11_get_default_xdisplay (),
+                           "Composite", &opcode, &event, &error);
+}