Ensure cursor position is correct when filling line buffer

kitty/screen.c

@@ -330,6 +330,7 @@ screen_resize(Screen *self, unsigned int lines, unsigned int columns) {
             if (!historybuf_pop_line(self->historybuf, self->alt_linebuf->line)) break;
             INDEX_DOWN;
             linebuf_copy_line_to(self->main_linebuf, self->alt_linebuf->line, 0);
+            self->cursor->y++;
         }
     }
     return true;

kitty_tests/screen.py

@@ -318,8 +318,10 @@ class TestScreen(BaseTest):
         # Height increased, width unchanged → pull down lines to fill new space at the top
         s = prepare_screen(map(str, range(6)))
         assert_lines('2', '3', '4', '5', '')
+        dist_from_bottom = s.lines - s.cursor.y
         s.resize(7, s.columns)
         assert_lines('0', '1', '2', '3', '4', '5', '')
+        self.ae(dist_from_bottom, s.lines - s.cursor.y)
 
         # Height increased, width increased → rewrap, pull down
         s = prepare_screen(['0', '1', '2', '3' * 15])