M-x flx completion breaks after using calc

[tigermonkey]

First reported at: lewang/flx#53

To reproduce problem:

M-x plp
[auto-completes to package-list-packages]
C-x * *
[open calc]
[input something]
M-x plp
[completes to pr-help]

If I don't input anything in the calc buffer, it still works.

Also, flx keeps working other completions, even smex-major-mode-commands just smex completion is broken.

If I load a minimal config I cannot reproduce it either.. even after eval-ing my config line by line I cannot narrow it to a particular line. I think it's related to mu4e, but it's hard to track down.

Do you have any suggestions on how to go about debugging it?

[nonsequitur]

I can't reproduce the issue with a blank Emacs 24.3.1 config that has only Smex installed.
Please provide additional details.

[tigermonkey]

Well at a minimum you'd need:

(require 'package)
(package-initialize)

(load-library "flx-ido")
(ido-mode t)
(flx-ido-mode 1)

(setq ido-use-faces nil)

(require 'smex)
(smex-initialize)
(global-set-key (kbd "M-x") 'smex)

But with that I cannot reproduce it, so I think it has something to do with a weird interaction between smex, calc, and something else.