My mother's PC had the most bizarre bug I've ever seen this morning.

PC specs: Windows 10 Home 64bit
Version: 1903 when problem occurred (1909 now)

What happened:
  • all desktop items, including folders, opened in Libreoffice Writer; once open it displayed a blank document then began flickering as if it had bugged while scrolling
  • opened Task Manager from the task bar, selecting the second tab opened the final tab, which began flickering like Libreoffice
  • problems stopped after a restart, then occurred again after another restart; then stopped after the PC ran for a short time
  • no error popups or BSODs
  • she said Candycrush reported an error earlier but it didn't occur when I looked at it; she said it mentioned "something about the desktop"

Steps I've taken:
  • did a quick virus scan then reboot scan with Windows Defender; no problems found
  • sfc /scannow; no problems found
  • installed update KB4537572 and upgraded system to 1909; updates appear to have been successful, no other updates available

At the moment the problem has disappeared, but since it reoccurred previously I'm not sure it hasn't been fixed.

I'm out of ideas. What else can I check and what other tests can I run? Based on the given information could anyone suggest what the problem might be?