We are talking same thing with different solutions.
At first I thought you want complete transparent (a ghost like) window. It is done.
but when I saw some samples at reddit, I understood your goal. you want a somekind of masking. Only content should be visible but not borders and other stuffs.
I went with SWindow solution, because I don’t want to put only a bitmap to that window. I wanted to put a whole UMG widget.
Vulkan can solve some problems but it is not an exact solution. For example, if you want to create a window for VR project, (I don’t speak about 3D widgets or VR interacted interfaces. I mean spectator/supervisor control windows for simulation or virtual production projects), you should use directx as target RHI.
Anyway, I am glad if you solve yout problem but I will meddling with this stuff just a little more 