>> I'd like to make it customizable by using the existing customization in
>> the arg WRITABLE of window-state-get and window-persistent-parameters,
>> even though formally prev/next-buffers is not a window parameter (maybe
>> it should be, I don't know).
>
> Always keep in mind that prev_buffers and next_buffers need special
> treatment in mark_object to make sure that dead buffers from window
> configurations stored somewhere (for example in a register) get their
> entries deleted and can be eventually reclaimed.  This is something I
> completely disregarded when writing the original code for navigating
> these buffer lists.  Stefan then wrote the code to do that in
> mark_object.

When testing the previous patch, I noticed that killed buffers automatically
disappear from prev/next-buffers lists, so there are no #<killed buffer> remains.
Is it handled by code in kill-buffer?

>> Done, with a small change: even though set-marker is idempotent in regard
>> to its POSITION arg (i.e. if POSITION is a marker, it creates an identical
>> marker), I added a check to not create a new one.  OTOH, get-buffer is
>> idempotent too, but it seems window-state-put never receives a structure
>> with buffer objects, and I'm not sure why window-state-get should always
>> use buffer-name regardless of the value WRITABLE, i.e. why should it return
>> buffer names as strings instead of buffer objects even when WRITABLE is nil?
>
> Maybe because I didn't care about non-writable states back then.  But
> you're obviously right.  Buffers can be renamed at will and in that
> case we might have a state with a non-existent buffer or even buffers
> with names switched and therefore invalid values of start or point.

What do you think about creating new functions to convert the existing
states from non-writable to writable and back?  Then in the same session
it would be more optimal to use window states with buffer/mark objects,
and states to save to the desktop could be serialized by such functions.

> So yes: Please use a buffer object for non-writable states.

Here is a new patch: