OS: KDE Dragonized Gaming Edition (latest update)
Kernel: 5.12.9-zen1-1-zen
Environment: KDE Plasma Garuda Dragonized
Description: After a prolonged use during the day, I have this weird bug when closing any window. As mention, when closed, animation of being vacuumed by Latte Dock starts playing and after that window is still visible as a part of the background (but over the icons) and it's non-interactive.
Also check the state of your compositor when these instances occur. Perhaps change your compositor settings as well as checking your ram usage when this occurs.
In the Compositor setting I have selected "Force smoothest animations", hit "Apply" and now the ghost windows disappeared. I have no problems with RAM usage as I have somewhat of a higher-end PC. In the past, in order to get rid of the annoying windows I had to restart,. So yes, I had restarted my computer after I have updated it.
Thank you all for the support and I will look up in the Wiki how to post the proper support questions in the future. Such welcoming community.