Image not hidden when switching between tmux sessions

[twidxuga]

Thanks for a great (and much relevant) project!

I noticed that, when switching between sessions in tmux, if there is an image preview in the current/active session, the image will not be hidden when switching to another session.

Switching windows within the same session does hide the image, as expected.

To reproduce:

1. Use ueberzugpp to display an image (e.g. ranger or with a script like the img script included in the source code), within a tmux session.
2. Switch to another running tmux session (e.g. with prefix+w and selecting the other session, or just prefix+w+<session number>)
3. Note that the image is not hidden, and is overlaid on top of whatever is being displayed in the active window of the new session.

Hope this helps. Happy to provide more info, test code patches, etc. as needed.

[twidxuga]

Just to add: I tested the original ueberzug, and it doesn't seem to have this issue.

[twidxuga]

Working on a patch, will create a pull request soon.

As it turns out, ueberzugpp is not tmux session aware when determining if the window is focussed, and doesn't set a global hook for client-session-changed, so it doesn't use that hook to hide the session.