window-list: Save and restore positions as runtime state

While it doesn't make sense for window list positions to be truly
persistent like dash items, some persistence is desirable.

Otherwise any manually set position is lost when the extension
is disabled, for example when locking the screen.

To address this, serialize the positions as runtime state on drop,
and restore them when populating the list.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell-extensions/-/merge_requests/338>

extensions/window-list/extension.js

@@ -35,6 +35,8 @@ const DRAG_RESIZE_DURATION = 400;
 
 const DRAG_PROXIMITY_THRESHOLD = 30;
 
+const SAVED_POSITIONS_KEY = 'window-list-positions';
+
 const GroupingMode = {
     NEVER: 0,
     AUTO: 1,
@@ -1095,6 +1097,8 @@ class WindowList extends St.Widget {
             for (let i = 0; i < apps.length; i++)
                 this._addApp(apps[i], false);
         }
+
+        this._restorePositions();
     }
 
     _updateKeyboardAnchor() {
@@ -1243,9 +1247,33 @@ class WindowList extends St.Widget {
 
         this._clearDragPlaceholder();
 
+        this._savePositions();
+
         return true;
     }
 
+    _getPositionStateKey() {
+        return `${SAVED_POSITIONS_KEY}:${this._monitor.index}`;
+    }
+
+    _savePositions() {
+        const buttons = this._windowList.get_children()
+            .filter(b => b instanceof BaseButton);
+        global.set_runtime_state(this._getPositionStateKey(),
+            new GLib.Variant('as', buttons.map(b => b.id)));
+    }
+
+    _restorePositions() {
+        const positions = global.get_runtime_state('as',
+            this._getPositionStateKey())?.deepUnpack() ?? [];
+
+        for (const button of this._windowList.get_children()) {
+            const pos = positions.indexOf(button.id);
+            if (pos > -1)
+                this._windowList.set_child_at_index(button, pos);
+        }
+    }
+
     _monitorItemDrag() {
         DND.addDragMonitor(this._itemDragMonitor);
     }