Bug#992870: transition: GNOME 40 (libmutter-8-0 and friends)

[Simon McVittie <smcv@debian.org>]

Package: release.debian.org
Severity: normal
Tags: moreinfo
User: release.debian.org@packages.debian.org
Usertags: transition
X-Debbugs-Cc: debian-gtk-gnome@lists.debian.org

We're heading towards the point where GNOME 40 is ready for unstable.
This involves the usual libmutter transition.

In addition, there have been various functionality moves and other
reshuffles among core GNOME packages. I think it will be best if we
deliberately *avoid* uploading some of the core GNOME packages to unstable
until we are fully ready for the transition.

We are *not* ready for a transition slot yet (hence moreinfo tag),
but I'm opening this bug early so we can mark it with its blockers.

Core packages:

- gsettings-desktop-schemas (must go first)
- gnome-settings-daemon
- gnome-control-center
- mutter
- gnome-shell
- gnome-desktop3
    - this one is not strictly versioned, but it'll be less confusing
      for everyone if we upload it as part of the same transaction
- budgie-desktop (non-GNOME package, fixed version in Ubuntu but not
  experimental)

Additionally, libgweather entangles the transition with:

- libgweather (has Breaks on lots of things)
- evolution-data-server :-(
- gnome-applets
- gnome-calendar
- gnome-panel
- gnome-weather
- wmforecast (non-GNOME package, fixed version in experimental)

Entanglement that I know about so far:

- libgweather has some sort of incompatible behaviour changes without
  a SONAME bump. I need to look into this. I hope this part can be
  broken out into a separate transition or something, perhaps by
  backporting support for the new libgweather into old callers or by
  temporarily adding compatibility with the old libgweather to new
  callers, because it links the mutter and evolution-data-server
  transitions and that doesn't seem ideal.

- Many new packages need the new gsettings-desktop-schemas.

- As usual, the new gnome-shell requires the new mutter, while the old
  gnome-shell requires the old mutter. We have to do this part in lockstep.

- GNOME Shell has changed its workspace layout from vertical to horizontal,
  and the default keybindings in gsettings-desktop-schemas have changed
  Super+PgUp, Super+PgDn to match. The new default keybindings will make
  no sense for the old Shell. I don't think this is necessarily important
  enough to need a Depends/Breaks, but we should minimize the time that
  this situation exists for.

- Mouse settings have moved from gnome-settings-daemon to
  gsettings-desktop-schemas. If we have the new g-s-d and the old g-d-s,
  things will still technically work, but gnome-tweaks will seem broken
  (because it's using the new settings that the old g-s-d ignores).

- Responsibility for audible feedback when taking a screenshot moved from
  gnome-settings-daemon to gnome-shell. Again, this isn't important enough
  to need Depends/Breaks, but we should minimize the skew.

- gnome-control-center configures all the other core packages so its
  version should not diverge.

Ben file for the mutter transition, which is the key thing here:

title = "mutter";
is_affected = .depends ~ "libmutter-7-0" | .depends ~ "libmutter-8-0";
is_good = .depends ~ "libmutter-8-0";
is_bad = .depends ~ "libmutter-7-0";