Layers view: restore expanded group layers after reload

Since they get collapsed due to the model reset.

todo: unduplicate code

src/tiled/layerdock.cpp

@@ -312,6 +312,8 @@ void LayerView::setMapDocument(MapDocument *mapDocument)
         auto layerModel = mMapDocument->layerModel();
         mProxyModel->setSourceModel(layerModel);
 
+        connect(mMapDocument, &MapDocument::changed,
+                this, &LayerView::changeEvent);
         connect(mMapDocument, &MapDocument::currentLayerChanged,
                 this, &LayerView::currentLayerChanged);
         connect(mMapDocument, &MapDocument::selectedLayersChanged,
@@ -377,6 +379,26 @@ void LayerView::indexPressed(const QModelIndex &proxyIndex)
         mMapDocument->setCurrentObject(layer);
 }
 
+void LayerView::changeEvent(const ChangeEvent &event)
+{
+    switch (event.type) {
+    case ChangeEvent::DocumentReloaded: {
+        // Restore expanded layers
+        const LayerModel *layerModel = mMapDocument->layerModel();
+        for (const int layerId : std::as_const(mMapDocument->expandedGroupLayers)) {
+            if (Layer *layer = mMapDocument->map()->findLayerById(layerId)) {
+                const QModelIndex sourceIndex = layerModel->index(layer);
+                const QModelIndex index = mProxyModel->mapFromSource(sourceIndex);
+                setExpanded(index, true);
+            }
+        }
+        break;
+    }
+    default:
+        break;
+    }
+}
+
 void LayerView::currentLayerChanged(Layer *layer)
 {
     const LayerModel *layerModel = mMapDocument->layerModel();

src/tiled/layerdock.h

@@ -106,6 +106,8 @@ class LayerView : public QTreeView
 
     void currentRowChanged(const QModelIndex &proxyIndex);
     void indexPressed(const QModelIndex &proxyIndex);
+
+    void changeEvent(const ChangeEvent &event);
     void currentLayerChanged(Layer *layer);
     void selectedLayersChanged();
     void layerRemoved(Layer *layer);