fix: chat input crashing when moving the cursor (#33)

Reviewed-on: #33

crates/ui/src/input/display_map/folding.rs

@@ -0,0 +1,96 @@
+use std::ops::Range;
+
+#[cfg(not(target_family = "wasm"))]
+use tree_sitter::Node;
+#[cfg(not(target_family = "wasm"))]
+pub use tree_sitter::Tree;
+
+#[cfg(target_family = "wasm")]
+/// Stub type for tree-sitter Tree on WASM (tree-sitter not available).
+pub struct Tree;
+
+#[cfg(not(target_family = "wasm"))]
+/// Minimum line span for a node to be considered foldable.
+const MIN_FOLD_LINES: usize = 2;
+
+/// A fold range representing a foldable code region.
+///
+/// The fold range spans from start_line to end_line (inclusive).
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
+pub struct FoldRange {
+    /// Start line (inclusive)
+    pub start_line: usize,
+    /// End line (inclusive)
+    pub end_line: usize,
+}
+
+impl FoldRange {
+    pub fn new(start_line: usize, end_line: usize) -> Self {
+        assert!(
+            start_line <= end_line,
+            "fold start_line must be <= end_line"
+        );
+        Self {
+            start_line,
+            end_line,
+        }
+    }
+}
+
+#[cfg(not(target_family = "wasm"))]
+/// Check if a named node qualifies as a fold candidate.
+///
+/// Uses a structural heuristic: any **named** node spanning ≥ MIN_FOLD_LINES
+/// is foldable. tree-sitter already parses code into semantic units (functions,
+/// classes, blocks, etc.), so named nodes naturally correspond to meaningful
+/// foldable regions across all languages without a per-language node-type list.
+fn is_foldable_node(node: &Node) -> bool {
+    let start = node.start_position().row;
+    let end = node.end_position().row;
+    end.saturating_sub(start) >= MIN_FOLD_LINES
+}
+
+#[cfg(not(target_family = "wasm"))]
+/// Extract fold ranges only within a byte range (for incremental updates after edits).
+///
+/// Skips subtrees entirely outside the range, making it O(nodes in range)
+/// instead of O(all nodes in tree).
+pub fn extract_fold_ranges_in_range(tree: &Tree, byte_range: Range<usize>) -> Vec<FoldRange> {
+    let mut ranges = Vec::new();
+    let root = tree.root_node();
+    let mut cursor = root.walk();
+    // Skip the root, it's not foldable. Use named_children to skip literal tokens.
+    for child in root.named_children(&mut cursor) {
+        collect_foldable_nodes_in_range(child, &byte_range, &mut ranges);
+    }
+
+    ranges.sort_by_key(|r| r.start_line);
+    ranges.dedup_by_key(|r| r.start_line);
+    ranges
+}
+
+#[cfg(not(target_family = "wasm"))]
+/// Recursively collect foldable nodes, skipping subtrees outside byte_range.
+fn collect_foldable_nodes_in_range(
+    node: Node,
+    byte_range: &Range<usize>,
+    ranges: &mut Vec<FoldRange>,
+) {
+    if node.end_byte() <= byte_range.start || node.start_byte() >= byte_range.end {
+        return;
+    }
+
+    if !is_foldable_node(&node) {
+        return;
+    }
+
+    ranges.push(FoldRange {
+        start_line: node.start_position().row,
+        end_line: node.end_position().row,
+    });
+
+    let mut cursor = node.walk();
+    for child in node.named_children(&mut cursor) {
+        collect_foldable_nodes_in_range(child, byte_range, ranges);
+    }
+}