Merge branch 'win-transparency'

Improve shaped sample and its documentation.

See #22230.

interface/wx/toplevel.h

@@ -585,6 +585,13 @@ public:
     /**
         If the platform supports it will set the window to be translucent.
 
+        Note that in wxGTK this function must be called before the window is
+        shown the first time it's called (but it can be called again after
+        showing the window too).
+
+        See @ref page_samples_shaped "the shaped sample" for an example of
+        using this function.
+
         @param alpha
             Determines how opaque or transparent the window will be, if the
             platform supports the operation. A value of 0 sets the window to be

interface/wx/window.h

@@ -2395,7 +2395,9 @@ public:
         Under wxGTK and wxOSX, you can use ::wxBG_STYLE_TRANSPARENT to obtain
         full transparency of the window background. Note that wxGTK supports
         this only since GTK 2.12 with a compositing manager enabled, call
-        IsTransparentBackgroundSupported() to check whether this is the case.
+        IsTransparentBackgroundSupported() to check whether this is the case,
+        see the example of doing it in @ref page_samples_shaped "the shaped
+        sample".
 
         Also, in order for @c SetBackgroundStyle(wxBG_STYLE_TRANSPARENT) to
         work, it must be called before Create(). If you're using your own