Fix generate_transaction_from_snippet to be less sensitive to selection

generate_transaction_from_snippet was previously re-using the original selection and it was making the core very sensitive to how that selection is located related to the text being replaced. This change updates the logic to create a new selection instead.

helix-lsp/src/lib.rs

@@ -289,10 +289,12 @@ pub mod util {
 
         // For each cursor store offsets for the first tabstop
         let mut cursor_tabstop_offsets = Vec::<SmallVec<[(i128, i128); 1]>>::new();
+        let mut replacement_end_selection = SmallVec::<[Range; 1]>::new();
         let transaction = Transaction::change_by_selection(doc, selection, |range| {
             let cursor = range.cursor(text);
             let replacement_start = (cursor as i128 + start_offset) as usize;
             let replacement_end = (cursor as i128 + end_offset) as usize;
+            replacement_end_selection.push(Range::point(replacement_end));
             let newline_with_offset = format!(
                 "{line_ending}{blank:width$}",
                 line_ending = line_ending,
@@ -323,15 +325,16 @@ pub mod util {
 
         // Create new selection based on the cursor tabstop from above
         let mut cursor_tabstop_offsets_iter = cursor_tabstop_offsets.iter();
-        let selection = selection
-            .clone()
+        let selection = Selection::new(replacement_end_selection, selection.primary_index())
             .map(transaction.changes())
             .transform_iter(|range| {
                 cursor_tabstop_offsets_iter
                     .next()
                     .unwrap()
                     .iter()
                     .map(move |(from, to)| {
+                        // replacement_end_selection is created from point ranges
+                        debug_assert!(range.anchor == range.head);
                         Range::new(
                             (range.anchor as i128 + *from) as usize,
                             (range.anchor as i128 + *to) as usize,