SUMMARY
After using color selector popup (shift + i), no keyboard shortcuts work until I switch to a different program and back to krita, or minimize then maximize the window. This problem was reported here:
https://linux.debian.bugs.dist.narkive.com/IPKwkFUI/bug-947361-krita-keyboard-doesn-t-respond-after-some-popups-f-e-show-color-selector-are-opened
The reporter said the bug was eventually fixed, but I have tried the latest stable build (4.4.2) and also the latest nightly builds (krita-4.4.3-alpha-c22b6d6-x86_64.appimage)(krita-5.0.0-prealpha-300d55c-x86_64.appimage) and it's present there. I tried a lot of older builds to narrow down when the bug was introduced to between 4.2.0 and 4.2.0-aplha (bug is present in all versions I tested after 4.2.0-aplha)
It was also reported here https://bugs.kde.org/show_bug.cgi?id=366353 but that report was closed whilst being unresolved (don't know if I'm allowed to reopen it). Apparently it might be related to XFCE and not a bug in Krita, if you think so I will report it to XFCE developers instead.

STEPS TO REPRODUCE
1. In a new file press shift + i to bring up the color selector popup
2. Move the cursor away to exit the popup
3. Try any keyboard shortcut (like shift + i again)

OBSERVED RESULT
Keyboard shortcuts not working (until switch out of Krita then back in)

EXPECTED RESULT
Keyboard shortcuts should work

SOFTWARE/OS VERSIONS
Linux: Xubuntu 20.04

ADDITIONAL INFORMATION
System information:

Krita

 Version: 4.4.2
 Languages: en_GB, en, en, en_GB, 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: 5.4.0-65-generic
  Pretty Productname: Ubuntu 20.04.2 LTS
  Product Type: ubuntu
  Product Version: 20.04
  Desktop: XFCE

OpenGL Info
 
  Vendor:  "NVIDIA Corporation" 
  Renderer:  "GeForce GTS 250/PCIe/SSE2" 
  Version:  "3.3.0 NVIDIA 340.108" 
  Shading language:  "3.30 NVIDIA via Cg compiler" 
  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 3.3, 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: 3.3
     Supports deprecated functions true 
     is OpenGL ES: false 

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

Hardware Information

  GPU Acceleration: desktop
  Memory: 7952 Mb
  Number of Cores: 8
  Swap Location: /tmp

Current Settings

  Current Swap Location: /tmp
  Current Swap Location writable: true
  Undo Enabled: true
  Undo Stack Limit: 30
  Use OpenGL: true
  Use OpenGL Texture Buffer: true
  Use AMD Vectorization Workaround: false
  Canvas State: OPENGL_SUCCESS
  Autosave Interval: 900
  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: 1
	Screen: 0
		Name: VGA-0
		Depth: 24
		Scale: 1
		Resolution in pixels: 1024x768
		Manufacturer: Sony
		Model: SDM-HS53-
		Refresh Rate: 60

Current Settings

  Current Swap Location: /tmp
  Current Swap Location writable: true
  Undo Enabled: true
  Undo Stack Limit: 30
  Use OpenGL: true
  Use OpenGL Texture Buffer: true
  Use AMD Vectorization Workaround: false
  Canvas State: OPENGL_SUCCESS
  Autosave Interval: 900
  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