Some windows suddenly become invisible
Every now and then, a window suddenly stops getting painted. It still exists in the "window buttons" and the "workspace switcher" in the panel, and even xdotool search --onlyvisible --name {some_name}
still can find it. In the "window buttons", I can show/hide it and it reflects in the "workspace switcher".
It began about 6 weeks ago, and it has happened mostly to Firefox windows (about twice a week), but also Remmina (2 times in total) and Geany (1 time, also in total). Most times, it happened when I was on another workspace and came back to the one the disappeared window was on.
I can focus the disappeared window using the "window buttons", and in case of Firefox, I can even switch tabs using Ctrl-PageDown, and the window button text changes accordingly.
Except the window not being painted, the application seems to work normally. So far, I have not found a way to make it visible again. Move to another workspace, "always on top", maximize, unmaximize, ... nothing helped and I needed to restart the application. Which is bad when I'm wiring a forum article in Firefox and can't save it or at least copy it to the clipboard. In Remmina I lose nothing, and in Geany, I can Shift-Crtl-S to "save all", but on a web page, all my input gets lost.
For the time being, a way to trigger repaint would help very much as a workaround.
I'm using XFCE on Arch linux, with all packages up-to-date.