bug#26104: 26.0.50; In Ubuntu, having mouse over other frame cause Alt key to produce a <switch-frame> event

[martin rudalics]

> 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