I don't really understand why we're exposing this. Is it for contained apps?

const QVariantMap map = { {QStringLiteral("result"), static_cast<uint>(result)} };

Will dbus look in autostart though?

context should be [this]

[this]

I'd call it windows, it's better not include the type. It will change to QVector with Qt6 anyway.

This is actually not API exposed to the clients, this is purely for xdg-desktop-portal needs, while the client interface (xdg-desktop-portal) has limited stuff available, see https://github.com/flatpak/xdg-desktop-portal/blob/master/data/org.freedesktop.portal.Background.xml.

Doesn't need to? I think it just specifies that the desktop file should ignore the exec line and start it through DBus (assuming correct dbus service file is installed etc.). Or am I wrong?

From the docs

Commandline to use add when autostarting at login.
     If this is not specified, the Exec line from the
     desktop file will be used.

I can't see us doing that last part

Doesn't need to? I think it just specifies that the desktop file should ignore the exec line and start it through DBus (assuming correct dbus service file is installed etc.). Or am I wrong?

You are right. We don't support it though.

The commandline is actually the Exec line, it's just named that way from the portal specification.

You can see below:

desktopEntryConfigGroup.writeEntry(QStringLiteral("Exec"), KShell::joinArgs(commandline));