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I remember kwin allows to fast switch to first window in window list, second, third, etc.

But I cannot found these shortcuts. Not in system settings and nowhere else.

Idea is simple: if behavior I've described do not exist, implement it, so pressing META+CTRL+2 will bring and activate second window on window list. META+CTRL+3 will do the same, but with third window on window list. Simple?

But.. we must known numbers. So shown it to users:
1. When pressing alt+tab
2. When pressing meta+ctrl

Also, last notification could be stored as second window on window list, when popup, so user could fast switch and handle notification message.

Thanks to that, I can order Firefox to download file, wait for popup telling file was downloaded and press META+CTRL+2, select open by keyboard and press enter.