[Enhancement] Avoid empty windows
It happens with both CSD and non-CSD windows. For example, when switching workspaces while the system load is high or the application is frozen, we see only the window border or nothing at all if it is CSD.
My suggestion is to fill these windows with the theme's main color or an old preview when there's nothing to show (may relate to the option to show previews on Alt+Tab). In case it's a CSD window, there could be a placeholder wm window.
Here's some examples: