I finally upgraded to 18.04LTS about two months ago and am still trying to acclimate myself to the GNOME 3.X DE. I noticed recently that the Alt-Tab task switcher doesn't update immediately with LibreOffice documents.
Case in point:
I've got LibreOffice, Firefox and a Nautilus window with a directory of LibreOffice spreadsheets open. When I Alt-Tab through this I see a generic LibreOffice icon, a Firefox icon (and the open windows) and a Nautilus icon. I'll then switch to the Nautilus window and open spreadsheet(s) and I see that LibreOffice has loaded them in the background behind the Nautilus window. Now when I Alt-Tab to switch to LibreOffice, there is not even a generic LibreOffice icon anymore. If I wait a few seconds and Alt-Tab again, I then see a LibreOffice Calc icon (and the open file windows).
Is this "normal" behaviour? I can't think of any other program that lags in the task switcher. I hope I was clear.