Question: This happens only in Skype on Ubuntu 18.04. I have used Skype on other platforms (Ubuntu and others) and this does not happen.
I use ALT+SHIFT to switch keyboard layout to another language. When I use Skype on Ubuntu 18.04 and press ALT+SHIFT the language is changed but also the focus is moved to the application menu (as if I had pressed ALT only).

Any help is welcome!

Answer: 
I guess the way you have set-up your ALT+SHIFT shortcut is via `gnome-tweak-tool`? 

If that's the case - I have the same issue with `slack`, after some digging found out that there is another way to change the keyboard layout w/o using `gnome-tweak-tool` which wont cause this side effect of losing the focus.

The solution is to use `gsettings` as explained [here](https://askubuntu.com/a/986629/567424)

```
gsettings set org.gnome.desktop.wm.keybindings switch-input-source "['<Shift>Alt_L']"
gsettings set org.gnome.desktop.wm.keybindings switch-input-source-backward "['<Alt>Shift_L']"

```

UPDATE: As mentioned in the comments, make sure to also uncheck the `ALT+SHIFT` shortcut from `gnome-tweak-tool`.
That was an odd issue. It looks like pressing only `Alt` is a Skype specific shortcut. `Alt`+`Shift`, OTOH, is an XKB shortcut. One way to avoid that Skype is affected is to make sure you press `Shift` first and then press `Alt` while still holding the `Shift` key.

Or, of course, using the default `Super`+`Space` is another way to avoid the conflict.