SUMMARY
When using the flood fill tool on a large area (like when trying to fill a huge layer) you can't cancel it.

STEPS TO REPRODUCE
1. Open a large document (e.g., 17k x 6x)
2. Click the flood fill tool
3. Check the settings (I'm using: fast mode off, threshold 52, grow 2 px, feathering 1 px, fill entire selection off, use selection as boundary off, sample current layer, pattern off)
4. Click in the layer to initiate a fill 
5. Try to cancel it by clicking the red circle cancel icon in the bottom status bar, or hitting escape, or hitting ctrl-c

OBSERVED RESULT
The flood fill doesn't cancel. Sometimes clicking the cancel button makes the progress bar disappear and reappear. Sometimes hitting escape or ctrl-c causes a dialog box to pop up saying "waiting for operation to complete: flood fill."

EXPECTED RESULT
Flood fill should cancel.

SOFTWARE/OS VERSIONS
Linux/KDE Plasma: Devuan Linux 


$ sha256sum krita-4.4.5-x86_64.appimage 
016f60e801507f8c232ae5892811cb45bee089245dc2b5e9b0a7f56c1e555c51  krita-4.4.5-x86_64.appimage


Krita

 Version: 4.4.5
 Languages: en_US, en
 Hidpi: true

Qt

  Version (compiled): 5.12.9
  Version (loaded): 5.12.9

OS Information

  Build ABI: x86_64-little_endian-lp64
  Build CPU: x86_64
  CPU: x86_64
  Kernel Type: linux
  Kernel Version: 4.19.0-14-amd64
  Pretty Productname: Devuan GNU/Linux 3 (beowulf)
  Product Type: devuan
  Product Version: 3
  Desktop: 

OpenGL Info
 
  Vendor:  "NVIDIA Corporation" 
  Renderer:  "GeForce GTX 970/PCIe/SSE2" 
  Version:  "4.6.0 NVIDIA 390.141" 
  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 

Hardware Information

  GPU Acceleration: auto
  Memory: 32107 Mb
  Number of Cores: 12
  Swap Location: /tmp

Current Settings

  Current Swap Location: /tmp
  Current Swap Location writable: true
  Undo Enabled: true
  Undo Stack Limit: 75
  Use OpenGL: true
  Use OpenGL Texture Buffer: true
  Use AMD Vectorization Workaround: false
  Canvas State: OPENGL_SUCCESS
  Autosave Interval: 180
  Use Backup Files: true
  Number of Backups Kept: 1
  Backup File Suffix: ~
  Backup Location: Same Folder as the File
  Backup Location writable: false
  Use Win8 Pointer Input: false
  Use RightMiddleTabletButton Workaround: false
  Levels of Detail Enabled: false
  Use Zip64: false


Display Information
Number of screens: 2
	Screen: 0
		Name: DP-0
		Depth: 24
		Scale: 1
		Resolution in pixels: 3840x1600
		Manufacturer: LG Electronics
		Model: LG HDR WQHD-
		Refresh Rate: 59
	Screen: 1
		Name: DP-4
		Depth: 24
		Scale: 1
		Resolution in pixels: 3840x2160
		Manufacturer: Wacom Tech
		Model: CintiqPro24P-
		Refresh Rate: 59