When saving a document, ProjectExplorerPanel.refresh() rebuilt the TreeView from scratch — resetting all nodes to collapsed — causing users to lose their position in the file hierarchy.

Changes

TreeExpansionState (new)

• Pure-Java generic utility decoupled from JavaFX, enabling unit testing without a JFX runtime
• Node<T> interface abstracts tree nodes; isCollectable() controls which nodes participate in state capture (directories only, by default)
• collectExpanded(Node<T>) / restoreExpanded(Node<T>, Set<T>) snapshot and replay expansion state

ProjectExplorerPanel

• refresh() now snapshots expanded dirs + selected file before rebuilding the tree, then restores both after
• asNode(TreeItem<File>) — private adapter bridging TreeItem<File> to TreeExpansionState.Node<File>; applies directory-only collection filter via isCollectable()

MarkNote

• saveCurrentDocument() calls revealFile() explicitly after refresh() to handle "Save As" (new file not yet in saved snapshot)

Tests & build

• TreeExpansionStateTest — 9 unit tests: null-safety, collect/restore semantics, and the full save→refresh round-trip
• build script updated to compile test sources against pre-built main classes (avoids recompiling JFX-dependent main sources in the test step)
• libs/junit-platform-console-standalone-6.0.1.jar added

// Before: all subdirs collapsed after every save
public void refresh() {
    TreeItem<File> rootItem = buildTreeItem(projectDir);
    rootItem.setExpanded(true);   // only root survives
    treeView.setRoot(rootItem);
}

// After: expanded state and selection survive the rebuild
public void refresh() {
    Set<File> expandedDirs = TreeExpansionState.collectExpanded(asNode(treeView.getRoot()));
    File selectedFile = /* current selection */;
    TreeItem<File> rootItem = buildTreeItem(projectDir);
    rootItem.setExpanded(true);
    treeView.setRoot(rootItem);
    TreeExpansionState.restoreExpanded(asNode(treeView.getRoot()), expandedDirs);
    if (selectedFile != null) revealFile(selectedFile);
}

• Fixes Issue when opening file, explorer tree is closed #52

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