Alt+F4 fails to exit
Using Alt+F4 when Vivaldi only shows the start page doesn't exit the program but reopens the last closed tab instead. Being a system-wide shortcut, it shouldn't be used for any other purpose than originally intended.
Which version are you using? It works for me with Snapshot 1.3.551.13 32-bit on Windows 10.
No shortcut is assigned to Exit in Settings, Keyboard, Window, Exit
Not the snapshot, the stable one. I have Alt+X assigned to Exit so that I can have something in those cases.
Then I wait for the forthcoming stable 13.