Fix shift change callbacks reading bad cursor (#11782)

Sloppy code inherited from Vim caused user scripts to be able
to observe the cursor line in an invalid intermediary state,
due to Neovim change callbacks being unbuffered unlike Vim listeners.

Manifested in Vimscript executed from the callback possibly erroring
when `:call`:ing any function,
due to the implicit range `curwin->w_cursor.lnum,curwin->w_cursor.lnum`
failing validation.

Fixed by deferring the call to `changed_lines()` until after
`curwin->w_cursor.lnum` gets its correct value.

1 files changed, 13 insertions, 0 deletions

diff --git a/test/functional/lua/buffer_updates_spec.lua b/test/functional/lua/buffer_updates_spec.lua
index 990cb97fec..7d3ecbf912 100644
--- a/test/functional/lua/buffer_updates_spec.lua
+++ b/test/functional/lua/buffer_updates_spec.lua
@@ -203,4 +203,17 @@ describe('lua: buffer event callbacks', function()
         { "test1", "lines", 1, tick+1, 5, 6, 5, 27, 20, 20 }}, exec_lua("return get_events(...)" ))
   end)
 
+  it('has valid cursor position while shifting', function()
+    meths.buf_set_lines(0, 0, -1, true, {'line1'})
+    exec_lua([[
+      vim.api.nvim_buf_attach(0, false, {
+        on_lines = function()
+          vim.api.nvim_set_var('listener_cursor_line', vim.api.nvim_win_get_cursor(0)[1])
+        end,
+      })
+    ]])
+    feed('>>')
+    eq(1, meths.get_var('listener_cursor_line'))
+  end)
+
 end)