Bug#1115704: gnome-session-common: doesn't prefer Papers/Showtime if Evince/Totem aren't installed

[Simon McVittie]

On Fri, 19 Sep 2025 at 13:00:23 +0300, Martin-Éric Racine wrote:
>pe 19.9.2025 klo 12.55 Simon McVittie (smcv at debian.org) kirjoitti:
>> The gnome-session-common in experimental already prefers Papers, but I
>> don't think we're ready to release that to unstable yet.

The reason I say this is because of the upstream code in gnome-session 
(which is entangled with components like gnome-shell and gdm), rather 
than because of the MIME apps list. It would be fine to prefer Papers 
with higher priority than Evince in unstable.

In stable, I think it would be more appropriate (certainly more 
conservative) to prefer Evince as higher priority than Papers, while 
still preferring Papers as higher priority than any other random app 
that happens to be able to open PDFs, such as GIMP.

>Stable/Unstable don't have to prefer the new apps. They merely have to
>acknowledge that Papers is a valid app for the same MIME types as
>Evince and Showtime for the same ones as Totem.

This is not about what is or is not a valid MIME handler. Whether Papers 
is a valid MIME handler for PDFs *is* under Papers' control (it's part 
of the .desktop file) and this is done correctly. mimeapps.list is about 
what the desktop environment prefers as a reasonable default. The only 
thing that can be done to make Papers a higher-priority PDF reader than 
GIMP is for the desktop environment to say that it prefers Papers > GIMP 
(in practice either Papers > Evince > other or Evince > Papers > other).

The problem is that Evince, Papers and GIMP all declare that they are 
valid PDF handlers, and in the absence of any information about which 
one to prefer, the desktop environment infrastructure will chose one 
essentially at random. I suspect it might be as simple as GIMP coming 
before Papers in alphabetical order, or perhaps gimp.desktop before 
org.gnome.Papers.desktop.

This is particularly visible for GIMP because nobody *actually* wants 
GIMP to be their default PDF reader (#525077, #759704), but it is 
completely true that it can read PDFs (therefore correct to list it as a 
MimeType), and there is no mechanism to mark a viewer/reader as 
less-preferred than others, only more-preferred.

Ideally there would be a way for GIMP to say "yes technically I can open 
PDFs, but it's never appropriate for this to be the default", so that 
random other PDF readers like xpdf would always "win" vs. GIMP; but there 
is currently no such mechanism in the freedesktop.org specifications 
(https://gitlab.freedesktop.org/xdg/xdg-specs/-/issues/219, 
https://gitlab.freedesktop.org/xdg/xdg-specs/-/issues/43) and until such 
a mechanism exists, it is not possible for GIMP to declare that it 
supports PDFs without it sometimes becoming the default.

     smcv