wallpaper: start wallp only once the burn's cover is on screen

The new wallpaper flashed in for a moment, the old one came back over
it, and only then did it burn. run() started wallp in the same turn as
swapping, while the cover was still mapping and decoding its two
pictures asynchronously, so swaybg swapped under an empty cover.

run() now holds wallp until each cover reports it is drawing (its
pictures decoded, then 50ms for a few frames), with a 1.5s fallback for
a cover that never comes: a missing screen or a picture that fails to
load. A run with nothing to cover starts at once, as before.

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

1 files changed, 11 insertions, 1 deletions

diff --git a/desktop/modules/appearance/WallpaperSpread.qml b/desktop/modules/appearance/WallpaperSpread.qml
index d1c383d..2f1dd03 100644
--- a/desktop/modules/appearance/WallpaperSpread.qml
+++ b/desktop/modules/appearance/WallpaperSpread.qml
@@ -73,7 +73,17 @@ Scope {
             // would pop in late over a half-drawn screen.
             readonly property bool ready: running && oldImg.status === Image.Ready
                 && newImg.status === Image.Ready
-            onReadyChanged: if (ready) anim.restart()
+            onReadyChanged: if (ready) {
+                anim.restart();
+                drawn.restart();
+            }
+            // A few frames for the cover to reach the screen, then wallp may
+            // swap swaybg underneath it.
+            Timer {
+                id: drawn
+                interval: 50
+                onTriggered: Wallpapers.covered()
+            }
 
             // -m fill is a centred crop, which is what PreserveAspectCrop does.
             Image {