Confirmed - I've seen this issue since Stretch.
How to replicate:
- Run a gui program
- Right-click on the program to open the menu for Raise, Restore, Maximize, etc.
- While that right-click menu is open, close the program using another means. It could be a program that takes some time to close, like maybe a half-frozen chromium or something, or you could set that program to close in a few seconds automatically using something like sleep 10 ; killall lxterminal.
The command lxpanelctl restart will unfreeze the panel, but good luck opening a terminal without a functional panel!
That's why I have guake terminal installed - just push F12.