>> Martin, could you suggest how to ignore window-sideness
>> while splitting the side window?  'tab-bar-new-tab-to'
>> currently uses such trick to create a new window:
>>
>>        (let ((ignore-window-parameters t))
>>          (delete-other-windows))
>
> Usually, you can't make a side window the only window on its frame - the
> main window has to stay.  If you ignore window parameters, you have to
> clean the state of windows yourself afterwards - for example, by giving
> the sole remaining window a 'window-side' parameter of nil.  If you do
> not do that, the windows state of the frame is broken already before you
> try splitting the window below.
>
>>        (unless (eq tab-bar-new-tab-choice 'window)
>>          ;; Create a new window to get rid of old window parameters
>>          ;; (e.g. prev/next buffers) of old window.
>>          (split-window) (delete-window))
>>
>> but this fails in side windows.
>
> This is a hack and would fail in the case at hand since 'split-window'
> would set the 'window-side' parameter for the new window.  I'd rather
> set all parameters of the selected window to nil instead.

Thanks, I confirm this fixes the problem with this patch.
Also ‘split-window’ and ‘delete-window’ were moved
inside ‘(ignore-window-parameters t)’ for more safety: