Due to overzealous user interface design Skype does not close with alt-F4 but instead minimises. That is not considered good behaviour.
I have found no setting in Skype to restore it normal Windows behaviour.
So I found and copied a Authotkey script that restores makes alt-F4 properly close Skype.
#IfWinActive ahk_exe Skype.exe ahk_class tSkMainForm
Kudos to original article https://autohotkey.com/board/topic/126066-script-to-run-skype-with-real-alt-f4/