SUMMARY
This may be a panels bug; I  don't know where to report it in that case. But I've only seen this happen in Dolphin, and I know the panels work fine while other long running jobs are running. 
Neon, user: plasma 5.15.1 

STEPS TO REPRODUCE
1. Set a kde panel to autohide (not necessary to reproduce, but makes it easier to see)
2. Execute a long running cut and paste action in Dolphin. Eg open a large tree such as unpacked Chromium source, cut all contents, move to the parent directory, and paste
3. Try to open the autohide panel. The frame appears, but there is no drawing of panel content until the paste operation finished. If it is a standard, always-visible panel, you will notice that contents do not update and it appears to have no interactivity

OBSERVED RESULT
see above

EXPECTED RESULT
panels should work normally, and even provide feedback about the long-running job 

SOFTWARE/OS VERSIONS


ADDITIONAL INFORMATION
Operating System: KDE neon 5.15
KDE Plasma Version: 5.15.1
KDE Frameworks Version: 5.55.0
Qt Version: 5.12.0
Kernel Version: 4.18.0-15-generic
OS Type: 64-bit
Processors: 8 × Intel® Core™ i7-8550U CPU @ 1.80GHz
Memory: 31.2 GiB of RAM