wallpaper: keep the pair the engine put up

The engine's ticks are light runs: udt-accent leaves GRUB, the homepage
and SDDM on the home pair. When a tick lands on a combination worth
keeping, there was no way to promote it short of re-picking both
images by hand.

"Keep this pair" reruns wallp on the current pair without
UDT_ROTATION, so the full run brings those three along, and makes it
the engine's home, so switching the engine off does not undo it. The
timer restarts, as after a pick by hand. The button shows only while
the engine is on and the pair up differs from home, so it hides once
kept.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

1 files changed, 14 insertions, 0 deletions

diff --git a/desktop/modules/appearance/WallEngine.qml b/desktop/modules/appearance/WallEngine.qml
index 54705f6..f8d96ac 100644
--- a/desktop/modules/appearance/WallEngine.qml
+++ b/desktop/modules/appearance/WallEngine.qml
@@ -135,6 +135,20 @@ Singleton {
         nextAt = Date.now() + timer.interval;
     }
 
+    // The pair up now differs from home only when a tick put it there, with
+    // the light run.
+    readonly property bool keepable: on
+        && (cfg.homeH !== Wallpapers.currentH || cfg.homeV !== Wallpapers.currentV)
+    // Liked what a tick put up: a full run of the same pair brings GRUB, the
+    // homepage and SDDM along, and it becomes home, so switching off does not
+    // undo that, and stays up a full interval like a pick by hand.
+    function keep() {
+        const h = Wallpapers.currentH, v = Wallpapers.currentV;
+        if (!keepable || !Wallpapers.run(h, v, "keep")) return;
+        Prefs.setWallEngine({ homeH: h, homeV: v });
+        restartTimer();
+    }
+
     // A switch-off during a run cannot start its own; it goes when that run
     // finishes.
     property bool homeWaiting: false