>>> But KILL means to delete the frame and the tab-bar code wants to prevent
>>> that.  Or what am I missing?
>>
>> For frames KILL defines whether to delete or iconify the frame.
>> There is no such thing as to "iconify" the tab.
>
> Don't you want to distinguish 'kill' non-nil where the frame should get
> deleted from 'kill' nil where nothing is done?  In the latter case the
> window cannot even be deleted because when the return value of
> 'window-deletable-p' equals 'frame' this implies that WINDOW is the root
> window of its frame.

This is the best that I can do, you can improve it if you want: