bug#34771: 27.0.50; object is a window sometimes although it should be a buffer in conditional display spec

[Tassilo Horn]

The docs state:

,----[ (info "(elisp)Other Display Specs") ]
|    You can make any display specification conditional.  To do that,
| package it in another list of the form ‘(when CONDITION . SPEC)’.  Then
| the specification SPEC applies only when CONDITION evaluates to a
| non-‘nil’ value.  During the evaluation, ‘object’ is bound to the string
| or buffer having the conditional ‘display’ property.  ‘position’ and
| ‘buffer-position’ are bound to the position within ‘object’ and the
| buffer position where the ‘display’ property was found, respectively.
| Both positions can be different when ‘object’ is a string.
`----

So in a conditional display spec, object should always be a buffer.
However, that doesn't seem to be the reality.  Sometimes, it is also a
window.

Here's a recipe for the current master:

$ emacs -q

In *scratch* insert and evaluate:

(insert (propertize "foobar" 'display
                    `(when (get-buffer-window object)
                       . ,(format "%s" (random)))) "\n")

Now move point over the displayed random number, hit and hold C-l, or
do whatever triggers redisplay.

Now I have this in *Messages*:

Error during redisplay: (eval (get-buffer-window object)) signaled 
(wrong-type-argument stringp #<window 3 on *scratch*>) [33 times]


In GNU Emacs 27.0.50 (build 8, x86_64-pc-linux-gnu, GTK+ Version 3.24.5)
 of 2019-03-06 built on thinkpad-t440p
Repository revision: 2260e48d2eeaa9ea2b9c17389cc25d00f85c414c
Repository branch: master
Windowing system distributor 'The X.Org Foundation', version 11.0.12004000
System Description: Arch Linux

Recent messages:
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20190306T182624.202> Expiring articles...done
20190306T182625.333> Saving Gnus registry (9449 entries) to 
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Saving file /home/horn/.gnus.d/.newsrc.eld...
Wrote /home/horn/.gnus.d/.newsrc.eld
20190306T182626.864> Saving /home/horn/.gnus.d/.newsrc.eld...done

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