API: fix cursor position when lines are added #9961

Restore code removed in #9674.

1 files changed, 21 insertions, 0 deletions

diff --git a/src/nvim/api/buffer.c b/src/nvim/api/buffer.c
index da4db60ad6..4a0b8d13d0 100644
--- a/src/nvim/api/buffer.c
+++ b/src/nvim/api/buffer.c
@@ -474,6 +474,7 @@ void nvim_buf_set_lines(uint64_t channel_id,
               false);
 
   changed_lines((linenr_T)start, 0, (linenr_T)end, (long)extra, true);
+  fix_cursor((linenr_T)start, (linenr_T)end, (linenr_T)extra);
 
 end:
   for (size_t i = 0; i < new_len; i++) {
@@ -1106,6 +1107,26 @@ free_exit:
   return 0;
 }
 
+// Check if deleting lines made the cursor position invalid.
+// Changed lines from `lo` to `hi`; added `extra` lines (negative if deleted).
+static void fix_cursor(linenr_T lo, linenr_T hi, linenr_T extra)
+{
+  if (curwin->w_cursor.lnum >= lo) {
+    // Adjust cursor position if it's in/after the changed lines.
+    if (curwin->w_cursor.lnum >= hi) {
+      curwin->w_cursor.lnum += extra;
+      check_cursor_col();
+    } else if (extra < 0) {
+      curwin->w_cursor.lnum = lo;
+      check_cursor();
+    } else {
+      check_cursor_col();
+    }
+    changed_cline_bef_curs();
+  }
+  invalidate_botline();
+}
+
 // Normalizes 0-based indexes to buffer line numbers
 static int64_t normalize_index(buf_T *buf, int64_t index, bool *oob)
 {