API: fix cursor position when lines are added #9961

Restore code removed in #9674.

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)
 {

test/functional/api/buffer_spec.lua

@@ -39,6 +39,41 @@ describe('api/buf', function()
       eq(1, curbuf_depr('line_count'))
     end)
 
+    it('cursor position is maintained after lines are inserted #9961', function()
+      -- replace the buffer contents with these three lines.
+      request('nvim_buf_set_lines', 0, 0, -1, 1, {"line1", "line2", "line3", "line4"})
+      -- Set the current cursor to {3, 2}.
+      curwin('set_cursor', {3, 2})
+
+      -- add 2 lines and delete 1 line above the current cursor position.
+      request('nvim_buf_set_lines', 0, 1, 2, 1, {"line5", "line6"})
+      -- check the current set of lines in the buffer.
+      eq({"line1", "line5", "line6", "line3", "line4"}, buffer('get_lines', 0, 0, -1, 1))
+      -- cursor should be moved below by 1 line.
+      eq({4, 2}, curwin('get_cursor'))
+
+      -- add a line after the current cursor position.
+      request('nvim_buf_set_lines', 0, 5, 5, 1, {"line7"})
+      -- check the current set of lines in the buffer.
+      eq({"line1", "line5", "line6", "line3", "line4", "line7"}, buffer('get_lines', 0, 0, -1, 1))
+      -- cursor position is unchanged.
+      eq({4, 2}, curwin('get_cursor'))
+
+      -- overwrite current cursor line.
+      request('nvim_buf_set_lines', 0, 3, 5, 1, {"line8", "line9"})
+      -- check the current set of lines in the buffer.
+      eq({"line1", "line5", "line6", "line8",  "line9", "line7"}, buffer('get_lines', 0, 0, -1, 1))
+      -- cursor position is unchanged.
+      eq({4, 2}, curwin('get_cursor'))
+
+      -- delete current cursor line.
+      request('nvim_buf_set_lines', 0, 3, 5, 1, {})
+      -- check the current set of lines in the buffer.
+      eq({"line1", "line5", "line6", "line7"}, buffer('get_lines', 0, 0, -1, 1))
+      -- cursor position is unchanged.
+      eq({4, 2}, curwin('get_cursor'))
+    end)
+
     it('line_count has defined behaviour for unloaded buffers', function()
       -- we'll need to know our bufnr for when it gets unloaded
       local bufnr = curbuf('get_number')