If the window is created as Eo, the alpha setting doesn't work.
Please check code https://pastebin.com/P33XtbMv. At line 99, you can choose between legacy and Eo for the window. With legacy, clicking on the "Alpha On" button sets the alpha, as expected. With Eo, it seems that something is tried, as the window "jumps" but nothing really happens.
May it be related to default visibility for Eo?