Created attachment 118930 [details]
Canvas transparency bug when the background is at 0% opacity

With a file that contains only one transparent layer (or if the background layer  is at 0% opacity), if you paint strokes on an empty layer, you will see through Krita's window everywhere the strokes are less than 100% opacity, revealing the windows in the background.

This happens only with OpenGL activated.

Steps:
1- Create a new standard file with 2 layers.
2- Delete the background layer or put it at 0% opacity.
3- Paint on the top layer with light strokes with a preset like "Basic 4 Flow Opacity"
4- Other windows behind Krita (or the desktop wallpaper) will show through the partially transparent strokes.

Version: 4.2.0-pre-alpha (git fbba406)

OS Information

  Build ABI: x86_64-little_endian-llp64
  Build CPU: x86_64
  CPU: x86_64
  Kernel Type: winnt
  Kernel Version: 10.0.17763
  Pretty Productname: Windows 10 (10.0)
  Product Type: windows
  Product Version: 10

OpenGL Info
 
  Vendor:  "NVIDIA Corporation" 
  Renderer:  "GeForce GTX 1080 Ti/PCIe/SSE2" 
  Version:  "4.6.0 NVIDIA 418.81" 
  Shading language:  "4.60 NVIDIA"