Fix ctrl-d/u issues with scroll_beyond_last_line off (#15395)

Closes #15356

Release Notes:

- vim: Fixed issues with `ctrl-d`/`ctrl-u` when
`scroll_beyond_last_line` is set to `off`
([#15356](https://github.com/zed-industries/zed/issues/15356)).


https://github.com/user-attachments/assets/d3166393-4a4e-4195-9db6-3ff1d4aeec78

---------

Co-authored-by: Marshall Bowers <elliott.codes@gmail.com>

crates/editor/src/scroll.rs

@@ -505,7 +505,7 @@ impl Editor {
         }
 
         if let Some(visible_lines) = self.visible_line_count() {
-            if newest_head.row() < DisplayRow(screen_top.row().0 + visible_lines as u32) {
+            if newest_head.row() <= DisplayRow(screen_top.row().0 + visible_lines as u32) {
                 return Ordering::Equal;
             }
         }