Description of a bug: When Besier Tool is used to draw a outline with a brush preset on a raster layer, from time to time canvas becomes 

unclickable (you can't draw anything) and some other functionality is lost. I only use a mouse, not a tablet, to draw with Besier Tool.
...
Other functionality lost:
Mosewheel zoom doesn't work.
Middle mouse button click-and-drag doesn't work.
Canvas rotate (keys 4,5 and 6) doesn't work.
When a 2nd document is created, and then got focused back on a 1st document, only a corner (small portion) of a canvas is visible, and a canvas 

window scrolling sliders are not functioning.
...
Most of commands applied from a menu does function.
Bug never occures while using other tools or Besier Tool with a fill, but no outline, or using Besier Tool with outline on a vector layer.
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00h.00m...........Krita started, new Document (single layer, 2000x3000px, 8bit color depth) created, starting to work with a Besier Tool continuing only with single layer.
00.20 Bug occured
00.27 Bug occured
01.22 Bug occured
01.32 Bug occured
01.36 Bug occured
02.20 Bug occured
02.24 Bug occured
...
Saving, closing and re-opening Document do not make the bug dissapear. Creating 2nd Document while first one is still opened doesn't work 

either. Restart of Krita is the only solution.
...
The only settings I changed from what I got as a default by install is >autosave time (15mins) and >undo stack size (10)
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Tablet used: Intuos CTH-480, Driver version: 6.3.21-10
...
Configuration data:

Krita
  Version: 4.0.0

OS Information
  Build ABI: x86_64-little_endian-llp64
  Build CPU: x86_64
  CPU: x86_64
  Kernel Type: winnt
  Kernel Version: 6.1.7601
  Pretty Productname: Windows 7 SP 1 (6.1)
  Product Type: windows
  Product Version: 7sp1

OpenGL Info 
  Vendor:  ATI Technologies Inc. 
  Renderer:  "ATI Mobility Radeon HD 4200 Series" 
  Version:  "3.0.11672 Compatibility Profile Context" 
  Shading language:  3.30 
  Requested format:  QSurfaceFormat(version 3.0, options QFlags<QSurfaceFormat::FormatOption>(DeprecatedFunctions), depthBufferSize 24, 

redBufferSize -1, greenBufferSize -1, blueBufferSize -1, alphaBufferSize -1, stencilBufferSize 8, samples -1, swapBehavior 

QSurfaceFormat::SwapBehavior(DoubleBuffer), swapInterval 0, profile  QSurfaceFormat::OpenGLContextProfile(CompatibilityProfile)) 
  Current format:    QSurfaceFormat(version 3.0, options QFlags<QSurfaceFormat::FormatOption>(DeprecatedFunctions), depthBufferSize 24, 

redBufferSize 8, greenBufferSize 8, blueBufferSize 8, alphaBufferSize 8, stencilBufferSize 8, samples 0, swapBehavior 

QSurfaceFormat::SwapBehavior(DoubleBuffer), swapInterval 1, profile  QSurfaceFormat::OpenGLContextProfile(NoProfile)) 
     Version: 3.0
     Supports deprecated functions true 
     is OpenGL ES: false 

QPA OpenGL Detection Info 
  supportsDesktopGL: true 
  supportsAngleD3D11: true 
  isQtPreferAngle: false 
  overridePreferAngle: false 
== log ==
 createPlatformOpenGLContext QSurfaceFormat(version 3.0, options QFlags<QSurfaceFormat::FormatOption>(DeprecatedFunctions), 

depthBufferSize 24, redBufferSize -1, greenBufferSize -1, blueBufferSize -1, alphaBufferSize -1, stencilBufferSize 8, samples -1, 

swapBehavior QSurfaceFormat::SwapBehavior(DoubleBuffer), swapInterval 0, profile  QSurfaceFormat::OpenGLContextProfile

(CompatibilityProfile))
Basic wglCreateContext gives version 3.3
OpenGL 2.0 entry points available
GPU features: QSet()
supportedRenderers GpuDescription(vendorId=0x1002, deviceId=0x9712, subSysId=0xfd3c1179, revision=0, driver: "aticfx64.dll", 

version=8.17.10.1129, "ATI Mobility Radeon HD 4200 Series") renderer:  QFlags(0x1|0x2|0x4|0x8|0x20)
Qt: Using WGL and OpenGL from "opengl32.dll"
create OpenGL: "ATI Technologies Inc.","ATI Mobility Radeon HD 4200 Series" default ContextFormat: v3.3 profile: 

QSurfaceFormat::OpenGLContextProfile(CompatibilityProfile) options: QFlags<QSurfaceFormat::FormatOption>

(DeprecatedFunctions),SampleBuffers, Extension-API present
Extensions: 188
"choosePixelFormat  Attributes:  0x2003 , 0x2027 , 0x2010 , 0x1 , 0x2001 , 0x1 , 0x2014 , 0x18 , 0x2011 , 0x1 , 0x2022 , 0x18 , 0x2013 , 

0x202b , 0x201b , 0x8 , 0x2023 , 0x8 , 0x2041 , 0x0 , \n    obtained px # 2  of  16 \n     PIXELFORMATDESCRIPTOR dwFlags=0x8025 

PFD_DRAW_TO_WINDOW PFD_SUPPORT_OPENGL PFD_SUPPORT_COMPOSITION PFD_DOUBLEBUFFER iPixelType=0 cColorBits=32 

cRedBits=8 cRedShift=16 cGreenBits=8 cGreenShift=8 cBlueBits=8 cBlueShift=0 cDepthBits=24 cStencilBits=8 cAuxBuffers=4 iLayerType=0 

cAlphaBits=8 cAlphaShift=24 "
createContext Creating context version 3 . 0 3 attributes
QWindowsGLContext 0x2011c50 ARB  requested:  QSurfaceFormat(version 3.0, options QFlags<QSurfaceFormat::FormatOption>

(DeprecatedFunctions), depthBufferSize 24, redBufferSize -1, greenBufferSize -1, blueBufferSize -1, alphaBufferSize -1, stencilBufferSize 8, 

samples -1, swapBehavior QSurfaceFormat::SwapBehavior(DoubleBuffer), swapInterval 0, profile  QSurfaceFormat::OpenGLContextProfile

(CompatibilityProfile)) 
    obtained # 2 ARB QSurfaceFormat(version 3.0, options QFlags<QSurfaceFormat::FormatOption>(DeprecatedFunctions), depthBufferSize 24, 

redBufferSize 8, greenBufferSize 8, blueBufferSize 8, alphaBufferSize 8, stencilBufferSize 8, samples 0, swapBehavior 

QSurfaceFormat::SwapBehavior(DoubleBuffer), swapInterval 1, profile  QSurfaceFormat::OpenGLContextProfile(NoProfile)) 
     PIXELFORMATDESCRIPTOR dwFlags=0x8025 PFD_DRAW_TO_WINDOW PFD_SUPPORT_OPENGL PFD_SUPPORT_COMPOSITION 

PFD_DOUBLEBUFFER iPixelType=0 cColorBits=32 cRedBits=8 cRedShift=16 cGreenBits=8 cGreenShift=8 cBlueBits=8 cBlueShift=0 

cDepthBits=24 cStencilBits=8 cAuxBuffers=4 iLayerType=0 cAlphaBits=8 cAlphaShift=24  swap interval:  0 
    default:  ContextFormat: v3.3 profile: QSurfaceFormat::OpenGLContextProfile(CompatibilityProfile) options: 

QFlags<QSurfaceFormat::FormatOption>(DeprecatedFunctions) 
    HGLRC= 0x30000
createPlatformOpenGLContext QSurfaceFormat(version 3.0, options QFlags<QSurfaceFormat::FormatOption>(DeprecatedFunctions), 

depthBufferSize 24, redBufferSize -1, greenBufferSize -1, blueBufferSize -1, alphaBufferSize -1, stencilBufferSize 8, samples -1, 

swapBehavior QSurfaceFormat::SwapBehavior(DoubleBuffer), swapInterval 0, profile  QSurfaceFormat::OpenGLContextProfile

(CompatibilityProfile))
"choosePixelFormat  Attributes:  0x2003 , 0x2027 , 0x2010 , 0x1 , 0x2001 , 0x1 , 0x2014 , 0x18 , 0x2011 , 0x1 , 0x2022 , 0x18 , 0x2013 , 

0x202b , 0x201b , 0x8 , 0x2023 , 0x8 , 0x2041 , 0x0 , \n    obtained px # 2  of  16 \n     PIXELFORMATDESCRIPTOR dwFlags=0x8025 

PFD_DRAW_TO_WINDOW PFD_SUPPORT_OPENGL PFD_SUPPORT_COMPOSITION PFD_DOUBLEBUFFER iPixelType=0 cColorBits=32 

cRedBits=8 cRedShift=16 cGreenBits=8 cGreenShift=8 cBlueBits=8 cBlueShift=0 cDepthBits=24 cStencilBits=8 cAuxBuffers=4 iLayerType=0 

cAlphaBits=8 cAlphaShift=24 "
createContext Creating context version 3 . 0 3 attributes
QWindowsGLContext 0x2011cf0 ARB  requested:  QSurfaceFormat(version 3.0, options QFlags<QSurfaceFormat::FormatOption>

(DeprecatedFunctions), depthBufferSize 24, redBufferSize -1, greenBufferSize -1, blueBufferSize -1, alphaBufferSize -1, stencilBufferSize 8, 

samples -1, swapBehavior QSurfaceFormat::SwapBehavior(DoubleBuffer), swapInterval 0, profile  QSurfaceFormat::OpenGLContextProfile

(CompatibilityProfile)) 
    obtained # 2 ARB QSurfaceFormat(version 3.0, options QFlags<QSurfaceFormat::FormatOption>(DeprecatedFunctions), depthBufferSize 24, 

redBufferSize 8, greenBufferSize 8, blueBufferSize 8, alphaBufferSize 8, stencilBufferSize 8, samples 0, swapBehavior 

QSurfaceFormat::SwapBehavior(DoubleBuffer), swapInterval 1, profile  QSurfaceFormat::OpenGLContextProfile(NoProfile)) 
     PIXELFORMATDESCRIPTOR dwFlags=0x8025 PFD_DRAW_TO_WINDOW PFD_SUPPORT_OPENGL PFD_SUPPORT_COMPOSITION 

PFD_DOUBLEBUFFER iPixelType=0 cColorBits=32 cRedBits=8 cRedShift=16 cGreenBits=8 cGreenShift=8 cBlueBits=8 cBlueShift=0 

cDepthBits=24 cStencilBits=8 cAuxBuffers=4 iLayerType=0 cAlphaBits=8 cAlphaShift=24  swap interval:  0 
    default:  ContextFormat: v3.3 profile: QSurfaceFormat::OpenGLContextProfile(CompatibilityProfile) options: 

QFlags<QSurfaceFormat::FormatOption>(DeprecatedFunctions) 
    HGLRC= 0x10001 
== end log ==