bug#25115: 26.0.50; ibuffer fails to show confirmation buffer when there are side windows

[Liu Hui]

Hello,

When there are side windows, ibuffer fails to show the comfirmation 
buffer for buffers to be killed.

1. emacs -Q & create a side window by
(pop-to-buffer "test"
               '(display-buffer-in-side-window
                 (side . right)
                 (width . 0.2)))
2. switch to another buffer and M-x ibuffer
3. press 'D', mark two or more buffers for killing
4. press 'x'

then an error 'ibuffer-confirm-operation-on: Cannot split side window or 
parent of side window' is issued.


In GNU Emacs 26.0.50.2 (x86_64-pc-linux-gnu, GTK+ Version 3.18.9)
 of 2016-12-03 built on lgw01-50
Windowing system distributor 'The X.Org Foundation', version 11.0.11804000
System Description:     Ubuntu 16.04.1 LTS