Created attachment 129223 [details]
Mask Problem .kra file for illustration

SUMMARY
This bug is not present in the 4.3.0 beta-2 appimage but is present in the 4.3.1 alpha (git 741ba2a) and the 5.0.0 prealpha (git ce3a1a3) appimages.

If a layer mask or masks are applied to a paint layer then they work but turning them off/on has no effect.

I attach a .kra file for illustration and investigation.

STEPS TO REPRODUCE
1. Apply one or more layer masks to a paint layer. Use Transparency, Filter or Transform Masks or any combination.
[Note that the masks have the expected effect.]
2. Turn off the visibility of any mask or masks.
[Note that this has no effect on the on-screen image.]
3. Turn off the visibility of the paint layer and turn off all masks then turn on the visibility of the paint layer.
[Note that the mask visibility setting is now correctly shown]
4. Repeat step 3. and do some combination of on/off for different masks.

OBSERVED RESULT
The change in mask visibility is only shown after the paint layer has it's visibility turned off then on.

For a transform mask, there is an artifact of the untransformed state that persists for a few seconds before disappearing.

For a gaussian blur filter mask, there is an artifact that does not disappear unless the gaussin filter mask is reapplied and the paint layer visibility is turned off then on.

EXPECTED RESULT
Changing the visibility of the masks should give a rapid change in on-screen appearance with no artifacts.

SOFTWARE/OS VERSIONS
Krita

 Version: 5.0.0-prealpha (git ce3a1a3)
 Languages: en_GB, en, en, en_GB, en
 Hidpi: false

Qt

  Version (compiled): 5.12.8
  Version (loaded): 5.12.8

OS Information

  Build ABI: x86_64-little_endian-lp64
  Build CPU: x86_64
  CPU: x86_64
  Kernel Type: linux
  Kernel Version: 4.19.0-9-amd64
  Pretty Productname: Debian GNU/Linux 10 (buster)
  Product Type: debian
  Product Version: 10
  Desktop: MATE

OpenGL Info
 
  Vendor:  "NVIDIA Corporation" 
  Renderer:  "GeForce GTX 750 Ti/PCIe/SSE2" 
  Version:  "4.6.0 NVIDIA 440.82" 
  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::DoubleBuffer, swapInterval 0, colorSpace QSurfaceFormat::DefaultColorSpace, profile  QSurfaceFormat::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::DoubleBuffer, swapInterval 0, colorSpace QSurfaceFormat::DefaultColorSpace, profile  QSurfaceFormat::CompatibilityProfile) 
     Version: 4.6
     Supports deprecated functions true 
     is OpenGL ES: false 

QPA OpenGL Detection Info 
  supportsDesktopGL: true 
  supportsOpenGLES: true 
  isQtPreferOpenGLES: false 

ADDITIONAL INFORMATION