When fido-mode is enabled, you need to use M-j to end the selection, not RET. This is similar to ido-mode, which was already special-cased for multi-occur and multi-isearch-buffers.

The attached patch fixes these two instances, however I’ve discovered two more bugs while testing it. For one, M-j does NOT work in multi-isearch-files, and I’m not sure why. It’s supposed to work; it works for the other two. So I didn’t update the prompt in that function (which, by the way is not special cased for ido-mode,

For two, the check for ido-mode is actually broken, but I don’t yet know how to fix it. It does (eq read-buffer-function #'ido-read-buffer), but actually nowadays (maybe this was different in the past), ido-mode doesn’t set read-buffer-function, unless you call ido-everywhere, and then it still doesn’t set it but overrides it with an advice, so that its value is #f(advice-wrapper :override nil ido-read-buffer), and this fails the eq test.

Anyway, I’d suggest applying this patch for now, and then figuring out how to fix the check for ido.

Thanks,

Andy