docs: Add some notes about managing graphical user sessions

This is work in progress and not finished yet. However, I hope to have
captured some of the key points that came up in previous discussions
with appropriate notes about things that still need to be defined.

I may revisit it later. Also, feel free to completely rewrite if the
format is not quite right.

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+---
+title: Desktop Environment Integration
+category: Concepts
+layout: default
+---
+
+# Desktop Environments
+
+NOTE: This document is a work-in-progress.
+
+## Single Graphical Session
+
+systemd only supports running one graphical session per user at a time.
+While this might not have always been the case historically, having multiple
+sessions for one user running at the same time is problematic.
+The DBus session bus is shared between all the logins, and services that are
+started must be implicitly assigned to the user's current graphical session.
+
+In principle it is possible to run a single graphical session across multiple
+logind seats, and this could be a way to use more than one display per user.
+When a user logs in to a second seat, the seat resources could be assigned
+to the existing session, allowing the graphical environment to present it
+is a single seat.
+Currently nothing like this is supported or even planned.
+
+## Pre-defined systemd units
+
+[`systemd.special(7)`](https://www.freedesktop.org/software/systemd/man/systemd.special.html)
+defines the `graphical-session.target` and `graphical-session-pre.target` to
+allow cross-desktop integration. Furthermore, systemd defines the three base
+slices `background`, `apps` and `session`.
+All units should be placed into one of these slices depending on their purposes:
+
+ * `session.slice`: Contains only processes essential to run the user's graphical session
+ * `apps.slice`: Contains all normal applications that the user is running
+ * `background.slice`: Useful for low-priority background tasks
+
+The purpose of this grouping is to assign different priorities to the
+applications.
+This could e.g. mean reserving memory to session processes,
+preferentially killing background tasks in out-of-memory situations
+or assinging different memory/CPU/IO priorities to ensure that the session
+runs smoothly under load.
+
+TODO: Will there be a default to place units into e.g. `apps.slice` by default
+rather than the root slice?
+
+## XDG standardization for applications
+
+To ensure cross-desktop compatibility and encourage sharing of good practices,
+desktop environments should adhere to the following conventions:
+
+ * Application units should follow the scheme `apps-<launcher>-<ApplicationID>-<RANDOM>.service`,
+   e.g. `apps-gnome-org.gnome.Evince-12345.service`,
+   `apps-flatpak-org.telegram.desktop-12345.service` or `apps-KDE-org.kde.okular-12345.service`.
+ * Using `.service` units instead of `.scope` units, i.e. allowing systemd to
+   start the process on behalf of the caller,
+   instead of the caller starting the process and letting systemd know about it,
+   is encouraged.
+ * If no application ID is available, the launcher should generate a reasonable
+   name when possible (e.g. using `basename(argv[0])`). This name must not
+   contain a `-` character.
+
+This has the following advantages:
+ * Using the `apps-<launcher>-` prefix means that the unit defaults can be
+   adjusted using desktop environment specific drop-in files.
+ * The application ID can be retrieved by stripping the prefix and postfix.
+   This in turn should map to the corresponding `.desktop` file when available
+
+TODO: Define the name of slices that should be used.
+This could be `apps-<launcher>-<ApplicationID>-<RANDOM>.slice`.
+
+TODO: Does it really make sense to insert the `<launcher>`? In GNOME I am
+currently using a drop-in to configure `BindTo=graphical-session.target`,
+`CollectMode=inactive-or-failed` and `TimeoutSec=5s`. I feel that such a
+policy makes sense, but it may make much more sense to just define a
+global default for all (graphical) applications.
+
+ * Should application lifetime be bound to the session?
+ * May the user have applications that do not belong to the graphical session (e.g. launched from SSH)?
+ * Could we maybe add a default `apps-.service.d` drop-in configuration?
+
+## XDG autostart integration
+
+To allow XDG autostart integration, systemd will ship a cross-desktop generator
+to create appropriate units for the autostart directory.
+Desktop Environments will be able to make use of this simply by starting the
+appropriate XDG related targets (representing e.g. content of the
+`$XDG_CURRENT_DESKTOP` environment variable to handle `OnlyShowIn/NotShowIn`).
+The names and ordering rules for these targets are to be defined.
+
+This generator will likely never support certain desktop specific extensions.
+One such example is the GNOME specific feature to bind a service to a settings
+variable.
+
+## Startup and shutdown best practices
+
+Question here are:
+
+ * Are there strong opinions on how the session-leader process should watch the user's session units?
+ * Should systemd/logind/… provide an integrated way to define a session in terms of a running *user* unit?
+ * Is having `gnome-session-shutdown.target` that is run with `replace-irreversibly` considered a good practice?