wallpaper: a screen can draw from a gallery folder instead of its hearts

Every wallpaper in the gallery is wide, so hearting a whole folder one
thumbnail at a time is the long way to say "this folder". Each screen
now has a source on the engine page: its hearted list, or one gallery
folder picked by chip, stored as the Wallpapers.groups key in Prefs
(folderH, folderV, "" for the hearts).

A folder that has left the gallery gives an empty pool, so that screen
is left alone rather than quietly falling back to its hearts. The
hearted list stays saved behind a folder source and comes back with the
Hearted chip. Switching source needs nothing from the shuffle: draw()
already drops bag entries that are no longer in the pool.

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

3 files changed, 48 insertions, 7 deletions

diff --git a/desktop/Prefs.qml b/desktop/Prefs.qml
index 9a9b0b7..8634f87 100644
--- a/desktop/Prefs.qml
+++ b/desktop/Prefs.qml
@@ -51,10 +51,11 @@ Singleton {
     property int settingsSidebar: 200
 
     // The wallpaper engine (modules/appearance/WallEngine.qml): on or off,
-    // the interval, a list of paths per screen, and each screen's home, the
-    // wallpaper switching off returns to. In here rather than in list files:
+    // the interval, a list of paths per screen, the gallery folder a screen
+    // draws from instead of its list ("" for the list), and each screen's
+    // home, the wallpaper switching off returns to. In here rather than in list files:
     // the lists can be empty, and an empty FileView write writes nothing.
-    readonly property var wallEngineDefaults: ({ on: false, minutes: 30, h: [], v: [], homeH: "", homeV: "" })
+    readonly property var wallEngineDefaults: ({ on: false, minutes: 30, h: [], v: [], folderH: "", folderV: "", homeH: "", homeV: "" })
     property var wallEngine: wallEngineDefaults
     function setWallEngine(changes) {
         const w = Object.assign({}, root.wallEngine, changes);
@@ -122,6 +123,7 @@ Singleton {
             on: we.on === true,
             minutes: Number.isInteger(we.minutes) && we.minutes >= 1 ? we.minutes : 30,
             h: paths(we.h), v: paths(we.v),
+            folderH: str(we.folderH), folderV: str(we.folderV),
             homeH: str(we.homeH), homeV: str(we.homeV),
         };
     }
diff --git a/desktop/modules/appearance/WallEngine.qml b/desktop/modules/appearance/WallEngine.qml
index 9d90245..fe45f7b 100644
--- a/desktop/modules/appearance/WallEngine.qml
+++ b/desktop/modules/appearance/WallEngine.qml
@@ -31,6 +31,18 @@ Singleton {
 
     function list(which) { return which === "V" ? cfg.v : cfg.h; }
     function listed(path, which) { return list(which).indexOf(path) >= 0; }
+    // A gallery folder by its Wallpapers.groups key, "" for the hearted list.
+    function folder(which) { return which === "V" ? cfg.folderV : cfg.folderH; }
+    function setFolder(which, key) { Prefs.setWallEngine(which === "V" ? { folderV: key } : { folderH: key }); }
+    // What a tick draws from. A folder that is no longer in the gallery
+    // gives nothing, so that screen is left alone rather than falling back
+    // to the list behind the user's back.
+    function pool(which) {
+        const key = folder(which);
+        if (key === "") return list(which).filter(present);
+        const g = Wallpapers.groups.find(g => g.key === key);
+        return g ? g.files : [];
+    }
     // ponytail: present means in the gallery's last scan, so a file deleted
     // since stays drawable until a rescan, and wallp then skips it.
     function present(path) { return Wallpapers.files.indexOf(path) >= 0; }
@@ -64,8 +76,8 @@ Singleton {
     // A screen whose list is empty, or whose pick is already up, is left out.
     function tick() {
         if (Wallpapers.applying) return;
-        const h = Shuffle.draw(cfg.h.filter(present), bagH, Wallpapers.currentH);
-        const v = Shuffle.draw(cfg.v.filter(present), bagV, Wallpapers.currentV);
+        const h = Shuffle.draw(pool("H"), bagH, Wallpapers.currentH);
+        const v = Shuffle.draw(pool("V"), bagV, Wallpapers.currentV);
         const ph = h.pick !== Wallpapers.currentH ? h.pick : "";
         const pv = v.pick !== Wallpapers.currentV ? v.pick : "";
         if (Wallpapers.run(ph, pv, "tick")) {
diff --git a/desktop/modules/appearance/WallEngineTab.qml b/desktop/modules/appearance/WallEngineTab.qml
index 95e2df2..caa01f4 100644
--- a/desktop/modules/appearance/WallEngineTab.qml
+++ b/desktop/modules/appearance/WallEngineTab.qml
@@ -67,24 +67,51 @@ Item {
                 required property var modelData
                 readonly property var paths: WallEngine.list(modelData.which)
                 readonly property string home: modelData.which === "V" ? WallEngine.cfg.homeV : WallEngine.cfg.homeH
+                readonly property string folder: WallEngine.folder(modelData.which)
+                readonly property var group: Wallpapers.groups.find(g => g.key === folder) ?? null
                 width: col.width
                 spacing: 8
 
                 Text {
-                    text: `${screen.modelData.name}: ${screen.paths.length} hearted`
+                    text: screen.modelData.name + ": "
+                        + (screen.folder === "" ? `${screen.paths.length} hearted`
+                           : screen.group ? `${screen.group.files.length} in ${screen.group.name}`
+                           : "the folder is gone, so this screen stays as it is")
                         + (WallEngine.on && screen.home !== "" ? `, home ${Wallpapers.label(screen.home)}` : "")
                     color: Theme.subtext
                     font { family: Theme.fontFamily; pixelSize: Theme.fontSize - 2 }
                 }
 
+                // Where the timer draws from: the hearts, or one gallery
+                // folder in their place.
+                Flow {
+                    width: col.width
+                    spacing: 6
+                    Tab {
+                        text: "Hearted"
+                        selected: screen.folder === ""
+                        onClicked: WallEngine.setFolder(screen.modelData.which, "")
+                    }
+                    Repeater {
+                        model: Wallpapers.groups
+                        Tab {
+                            required property var modelData
+                            text: modelData.name
+                            selected: screen.folder === modelData.key
+                            onClicked: WallEngine.setFolder(screen.modelData.which, modelData.key)
+                        }
+                    }
+                }
+
                 Text {
-                    visible: screen.paths.length === 0
+                    visible: screen.folder === "" && screen.paths.length === 0
                     text: "Heart a wallpaper on the Wallpaper page, with this screen selected, to add it."
                     color: Theme.overlay
                     font { family: Theme.fontFamily; pixelSize: Theme.fontSize - 4 }
                 }
 
                 Flow {
+                    visible: screen.folder === ""
                     width: col.width
                     spacing: 8