I attach a patch for this bug that takes the simple approach of adding
SELECTION_REQUEST_EVENT to the list of events to ignore in
kbd_buffer_store_buffered_event.

This seems simple and safe (it's not something one would normally consider
an input event).

Further, at present it would not help helm to implement Eli's suggestion of
a list of events for input-pending-p to ignore, as Helm currently does not
use that (it has a custom version of while-no-input that does not call
input-pending-p).

As a helm user with an external clipboard manager, I was experiencing this
bug. I'm using Emacs with this patch currently; it seems fine, and I don't
have the Helm problem any more.

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