desktop-entry-spec: Add "SingleMainWindow" key

This is an upstreaming of the GNOME-specific X-GNOME-SingleWindow key, which functions as the same thing, signaling that an app only supports having a single one of its main window open.

We recently added support for apps with that key in KDE [1] and thought it would be useful to upstream (with a slightly more accurate name, since technically single-instance apps can open multiple windows) so that everyone could benefit from the same thing.

[1] https://invent.kde.org/plasma/plasma-workspace/-/merge_requests/1309

CI seems to be broken:

fatal: unable to access 'https://gitlab.freedesktop.org/ngraham/xdg-specs.git/': The requested URL returned error: 504

That URL works fine if you omit the trailing '

which functions as the same thing, signaling that an app only supports running as a single instance.

It doesn't, X-GNOME-SingleWindow indicates that you can't open another window. Whilst that would somewhat imply single instance, it isn't the same thing.

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• cb37ac1e - desktop-entry-spec: Add "SingleMainWindow" key

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changed the description

changed title from desktop-entry-spec: Add "SingleInstance" key to desktop-entry-spec: Add "SingleMainWindow" key

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• 3ea3bc26 - desktop-entry-spec: Add "SingleMainWindow" key

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cc @faure

merged

I have opened merge requests to support this in GNOME and Plasma:

• GNOME: https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2084
• Plasma: https://invent.kde.org/plasma/plasma-workspace/-/merge_requests/1334

mentioned in commit xrdesktop/gnome-shell@7012d08b