> In Ubuntu, if I have two frames open and the mouse is positioned
over the other frame (i.e. not over the active one), pressing the Alt
key produces a <switch-frame> event.
>
> While this event does nothing in itself, it disrupts things like
yank-pop, which no longer works if it is bound to M-y as usual
(because instead of the command sequence yank -> yank-pop, we now have
yank -> handle-switch-frame -> yank-pop, which gives an error
"user-error: Previous command was not a yank").
>
> To be honest, I don't know enough about X-Windows to know if this is
emacs' fault or x-windows fault. But it's very annoying.
>
> [Tried on Emacs versions 24.5, 25.1, and the 26.0 and Ubuntu 16.10.]
Could you try in simple.el to replace
(if (not (eq last-command 'yank))
(user-error "Previous command was not a yank"))
with
(if (not (memq last-command '(yank handle-switch-frame)))
(user-error "Previous command was not a yank"))
Thanks, martin