Doesn't get the real width of window when using with olivetti mode

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Hi, dear developers.

When using with olivetti mode, olivetti mode will change the value of (window-width root-win).
For example, while the true value of the window is 120, it may reduce it to 82.

As a result, when using something like

(push '(" *undo-tree*" :width 0.3 :position right) popwin:special-display-config)

popwin get the wrong width, and the popwin window is too large than it should be.

I have open an issue here too. rnkn/olivetti#87