Keep channels the user collapsed collapsed

Every tree rebuild, which any join, leave or talk-power change causes,
expanded every row again, so a collapsed channel reopened within
seconds. The tree now remembers which channels were collapsed and
opens everything else; reopening one restores what is below it. While
the find bar filters, everything opens so every match shows.

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

ts3-client/swing/src/main/java/com/ts3client/ui/ServerTreePanel.java

@@ -18,6 +18,8 @@ import javax.swing.JViewport;
 import javax.swing.KeyStroke;
 import javax.swing.SwingUtilities;
 import javax.swing.plaf.basic.BasicTreeUI;
+import javax.swing.event.TreeExpansionEvent;
+import javax.swing.event.TreeExpansionListener;
 import javax.swing.tree.DefaultMutableTreeNode;
 import javax.swing.tree.DefaultTreeModel;
 import javax.swing.tree.TreePath;
@@ -26,7 +28,9 @@ import java.awt.Rectangle;
 import java.awt.event.ActionEvent;
 import java.awt.event.MouseAdapter;
 import java.awt.event.MouseEvent;
+import java.util.HashSet;
 import java.util.List;
+import java.util.Set;
 
 /**
  * The server view: a tree of channels each containing its clients, styled to
@@ -159,8 +163,13 @@ public final class ServerTreePanel extends JPanel {
             if (!nickname.isEmpty()) actions.renameSelf(nickname);
         }
     };
-    /** True while {@link #rebuild()} clears and restores the selection, to swallow the transient null in between. */
+    /**
+     * True while the tree is changed programmatically: a {@link #rebuild()} clearing and restoring
+     * the selection, or channels being reopened. The listeners ignore what happens meanwhile.
+     */
     private boolean rebuilding;
+    /** Channels the user collapsed; every other channel is shown open. */
+    private final Set<Integer> collapsedChannels = new HashSet<>();
     private final ServerModel model;
     private final GroupIcons groupIcons;
     private final Actions actions;
@@ -225,6 +234,30 @@ public final class ServerTreePanel extends JPanel {
         tree.setDropMode(DropMode.ON_OR_INSERT);
         tree.setTransferHandler(new ServerTreeDragAndDrop(model, actions, tree, this::pathOf).transferHandler());
 
+        tree.addTreeExpansionListener(new TreeExpansionListener() {
+            @Override
+            public void treeCollapsed(TreeExpansionEvent e) {
+                if (rebuilding) return;
+                Object obj = ((DefaultMutableTreeNode) e.getPath().getLastPathComponent()).getUserObject();
+                if (obj instanceof ChannelNode) collapsedChannels.add(((ChannelNode) obj).id);
+            }
+
+            @Override
+            public void treeExpanded(TreeExpansionEvent e) {
+                if (rebuilding) return;
+                DefaultMutableTreeNode node = (DefaultMutableTreeNode) e.getPath().getLastPathComponent();
+                if (!(node.getUserObject() instanceof ChannelNode)) return;
+                collapsedChannels.remove(((ChannelNode) node.getUserObject()).id);
+                // A rebuild while it was shut left everything below it shut too.
+                rebuilding = true;
+                try {
+                    expandOpenChannels(node, e.getPath());
+                } finally {
+                    rebuilding = false;
+                }
+            }
+        });
+
         tree.addTreeSelectionListener(e -> {
             if (rebuilding) return;
             TreePath path = tree.getSelectionPath();
@@ -364,9 +397,7 @@ public final class ServerTreePanel extends JPanel {
                 if (node != null) root.add(node);
             }
             treeModel.reload();
-            for (int i = 0; i < tree.getRowCount(); i++) {
+            expandOpenChannels(root, new TreePath(root));
-                tree.expandRow(i);
-            }
             if (selected != null) {
                 TreePath path = pathOf(selected);
                 if (path != null) tree.setSelectionPath(path);
@@ -381,6 +412,21 @@ public final class ServerTreePanel extends JPanel {
         search.treeRebuilt();
     }
 
+    /**
+     * Opens {@code node} and every channel below it, except the ones the user collapsed and
+     * whatever is below those. While the find bar filters, everything opens, so every match shows.
+     */
+    private void expandOpenChannels(DefaultMutableTreeNode node, TreePath path) {
+        tree.expandPath(path);
+        boolean filtering = search.isFiltering();
+        for (int i = 0; i < node.getChildCount(); i++) {
+            DefaultMutableTreeNode child = (DefaultMutableTreeNode) node.getChildAt(i);
+            if (child.isLeaf() || !(child.getUserObject() instanceof ChannelNode)) continue;
+            if (!filtering && collapsedChannels.contains(((ChannelNode) child.getUserObject()).id)) continue;
+            expandOpenChannels(child, path.pathByAddingChild(child));
+        }
+    }
+
     private Object selectedUserObject() {
         TreePath path = tree.getSelectionPath();
         return path == null ? null : ((DefaultMutableTreeNode) path.getLastPathComponent()).getUserObject();
@@ -408,6 +454,7 @@ public final class ServerTreePanel extends JPanel {
     public void showDisconnected() {
         root.setUserObject("Not connected");
         root.removeAllChildren();
+        collapsedChannels.clear();
         treeModel.reload();
         search.treeRebuilt();
     }