Set the translucency of the top-level window containing this widget. So, e.g., if you call this on the widget for an IFRAME, the top level browser window containing the IFRAME actually gets set. Be careful.
This can fail if the platform doesn't support transparency/translucency. By default widgets are not transparent. This will also fail if the toplevel window is not a Mozilla window, e.g., if the widget is in an embedded context.
After translucency has been enabled, the initial alpha channel value for all pixels is 1, i.e., opaque. If the window is resized then the alpha channel values for all pixels are reset to 1.