I just saw that UMG actually has the feature I need, namely the Viewport Widget.
Unfortunately it completely rages on my video memory.
I then realized that I was using the DebugGame Editor configuration and switched back to Development Editor.
That at least prevented my screen from going black and instead showing an error message:
D:\BuildFarm\buildmachine_++depot+UE4-Releases+4.4\Engine\Source\Runtime\Windows\D3D11RHI\Private\D3D11Util.cpp(217): Fatal error:
Direct3DDevice->CreateTexture2D(TextureDesc,SubResourceData,OutTexture2D) failed
at D:\BuildFarm\buildmachine_++depot+UE4-Releases+4.4\Engine\Source\Runtime\Windows\D3D11RHI\Private\D3D11Texture.cpp:433
with error E_OUTOFMEMORY,
Size=1024x1024x1 Format=DXGI_FORMAT_R16G16B16A16_FLOAT(0x0000000A), NumMips=1, Flags=D3D11_BIND_RENDER_TARGET D3D11_BIND_SHADER_RESOURCE
This error occurs while dragging the Viewport Widget.
So I guess I have to wait for a newer version of UMG or until it goes final.
I will also open a bug report about this.