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

[martin rudalics]

> Steps to reproduce:
>
> 1. emacs -Q
> 2. eval the following code in *scratch* to create a side window.
> (pop-to-buffer "test"
>                 '(display-buffer-in-side-window
>                   (side . right)
>                   (width . 0.2)))
> 3. Now the side window has been created and selected. Press 'C-x o' to switch 
to the other window in the left, which displays *scratch*.
> 4. M-x ibuffer. Then press 'd' to mark two or more buffers for killing.
> 5. Press 'x'. Then an error 'ibuffer-confirm-operation-on: Cannot split side 
window or parent of side window' is issued.

Confirmed.  ‘ibuffer-confirm-operation-on’ selects the last window of
the selected frame's ‘window-list’ and tries to split that window.  In
your scenario the last window is the side window and cannot be split by
‘split-window’.

I pushed a fix for that on master, please have a look.

But someone who's using ibuffer on a regular basis should really try to
make it use ‘display-buffer’ instead.  There is absolutely no evidence
that the last window of a frame can be split more easily than the one
used by ibuffer itself and if no window can be split some other window
should be reused for displaying the buffer list.

Thanks again, martin