vim-patch:9.2.0384: stale Insstart after cursor move breaks undo (#39290)

vim-patch:9.2.0384: stale Insstart after <Cmd> cursor move breaks undo

Problem:  A <Cmd> command executed from Insert mode can sync undo and
          move the cursor before the next edit. stop_arrow() saved the
          new cursor line for undo, but left Insstart at the previous
          insertion point. A line-start backspace could then delete
          lines above the saved line without saving the joined range,
          leaving a pending undo entry whose bottom resolved above
          its top and raising E340.
Solution: Update Insstart and Insstart_textlen after the pending undo
          save so the next edit starts from the command-updated cursor
          position (Jaehwang Jung).

closes: vim/vim#20031

AI-assisted: Codex

https://github.com/vim/vim/commit/d4fb31762ea0b9de6fffb529c4ffee509621f74c

Co-authored-by: Jaehwang Jung <tomtomjhj@gmail.com>

src/nvim/edit.c

@@ -2206,6 +2206,9 @@ int stop_arrow(void)
     new_insert_skip = 2;
   } else if (ins_need_undo) {
     if (u_save_cursor() == OK) {
+      // A command or event may have moved the cursor after syncing undo.
+      Insstart = curwin->w_cursor;
+      Insstart_textlen = (colnr_T)linetabsize_str(get_cursor_line_ptr());
       ins_need_undo = false;
     }
   }

test/old/testdir/test_undo.vim

@@ -923,5 +923,19 @@ func Test_restore_cursor_position_after_undo()
   bw!
 endfunc
 
+func Test_undo_line_backspace_after_insert_cmd_cursor_movement()
+  new
+  setlocal backspace=eol undolevels=100
+  call setline(1, ['', '', 'abc', 'def'])
+  call cursor(4, 1)
+
+  let v:errmsg = ''
+  call feedkeys("i\<Cmd>setlocal undolevels=101 | call cursor(3, 1)\<CR>"
+        \ .. "\<BS>\<BS>\<Esc>u", 'xt')
+
+  call assert_equal('', v:errmsg)
+  call assert_equal(['', '', 'abc', 'def'], getline(1, '$'))
+  bwipe!
+endfunc
 
 " vim: shiftwidth=2 sts=2 expandtab