Created attachment 120562 [details]
Example for alpha issue

SUMMARY
When I copen a new Canvas filled with only alpha my desktop and other applications behind Krita shine though the canvas when drawing very lightly.
When creating a new layer below the issue is resolved. How ever when hiding the new layer the issue persist.


STEPS TO REPRODUCE
1. Create new empty (aka filled with alpha) canvas
2. draw lightly with a brush on a the canvas


OBSERVED RESULT
Desktop background appears in drawing


EXPECTED RESULT
Checkerboard pattern is persistent behind alpha surfaces


SOFTWARE/OS VERSIONS
OpenGL Info
 
  Vendor:  "NVIDIA Corporation" 
  Renderer:  "GeForce GTX 1060 6GB/PCIe/SSE2" 
  Version:  "4.6.0 NVIDIA 418.56" 
  Shading language:  "4.60 NVIDIA" 
  Requested format:  QSurfaceFormat(version 3.0, options QFlags<QSurfaceFormat::FormatOption>(DeprecatedFunctions), depthBufferSize 24, redBufferSize 8, greenBufferSize 8, blueBufferSize 8, alphaBufferSize 8, stencilBufferSize 8, samples -1, swapBehavior QSurfaceFormat::SwapBehavior(DoubleBuffer), swapInterval 0, profile  QSurfaceFormat::OpenGLContextProfile(CompatibilityProfile)) 
  Current format:    QSurfaceFormat(version 4.6, options QFlags<QSurfaceFormat::FormatOption>(DeprecatedFunctions), depthBufferSize 24, redBufferSize 8, greenBufferSize 8, blueBufferSize 8, alphaBufferSize 8, stencilBufferSize 8, samples -1, swapBehavior QSurfaceFormat::SwapBehavior(DoubleBuffer), swapInterval 0, profile  QSurfaceFormat::OpenGLContextProfile(CompatibilityProfile)) 
     Version: 4.6
     Supports deprecated functions true 
     is OpenGL ES: false 

QPA OpenGL Detection Info 
  supportsDesktopGL: true 
  supportsOpenGLES: true 
  isQtPreferOpenGLES: false 
== log ==
 Supported renderers: QFlags(0x2|0x4) 
Surface format preference list: 
* QSurfaceFormat(version 3.0, options QFlags<QSurfaceFormat::FormatOption>(DeprecatedFunctions), depthBufferSize 24, redBufferSize 8, greenBufferSize 8, blueBufferSize 8, alphaBufferSize 8, stencilBufferSize 8, samples -1, swapBehavior QSurfaceFormat::SwapBehavior(DoubleBuffer), swapInterval 0, profile  QSurfaceFormat::OpenGLContextProfile(CompatibilityProfile)) 
    QSurfaceFormat::RenderableType(OpenGL) 
* QSurfaceFormat(version 3.0, options QFlags<QSurfaceFormat::FormatOption>(DeprecatedFunctions), depthBufferSize 24, redBufferSize 8, greenBufferSize 8, blueBufferSize 8, alphaBufferSize 8, stencilBufferSize 8, samples -1, swapBehavior QSurfaceFormat::SwapBehavior(DoubleBuffer), swapInterval 0, profile  QSurfaceFormat::OpenGLContextProfile(CompatibilityProfile)) 
    QSurfaceFormat::RenderableType(OpenGLES) 
Probing format... QSurfaceFormat::RenderableType(OpenGL) 
Found format: QSurfaceFormat(version 3.0, options QFlags<QSurfaceFormat::FormatOption>(DeprecatedFunctions), depthBufferSize 24, redBufferSize 8, greenBufferSize 8, blueBufferSize 8, alphaBufferSize 8, stencilBufferSize 8, samples -1, swapBehavior QSurfaceFormat::SwapBehavior(DoubleBuffer), swapInterval 0, profile  QSurfaceFormat::OpenGLContextProfile(CompatibilityProfile)) 
    QSurfaceFormat::RenderableType(OpenGL) 

Krita

 Version: 4.2.0
 Languages: de
 Hidpi: false

Qt

  Version (compiled): 5.9.5
  Version (loaded): 5.9.5

OS Information

  Build ABI: x86_64-little_endian-lp64
  Build CPU: x86_64
  CPU: x86_64
  Kernel Type: linux
  Kernel Version: 4.15.0-47-generic
  Pretty Productname: Ubuntu 18.04.2 LTS
  Product Type: ubuntu
  Product Version: 18.04


Hardware Information

  GPU Acceleration: auto
  Memory: 15997 Mb
  Number of Cores: 4
  Swap Location: /tmp


ADDITIONAL INFORMATION
See attached images