patch 8.2.4984: dragging statusline fails for window with winbar

Problem:    Dragging statusline fails for window with winbar.
Solution:   Fix off-by-one error. (closes #10448)
diff --git a/src/testdir/test_winbar.vim b/src/testdir/test_winbar.vim
index 92da312..7972bc3 100644
--- a/src/testdir/test_winbar.vim
+++ b/src/testdir/test_winbar.vim
@@ -161,4 +161,32 @@
   call delete('XtestWinbarNotVisble')
 endfunction
 
+func Test_drag_statusline_with_winbar()
+  call SetupWinbar()
+  let save_mouse = &mouse
+  set mouse=a
+  set laststatus=2
+
+  call test_setmouse(&lines - 1, 1)
+  call feedkeys("\<LeftMouse>", 'xt')
+  call test_setmouse(&lines - 2, 1)
+  call feedkeys("\<LeftDrag>", 'xt')
+  call assert_equal(2, &cmdheight)
+
+  call test_setmouse(&lines - 2, 1)
+  call feedkeys("\<LeftMouse>", 'xt')
+  call test_setmouse(&lines - 3, 1)
+  call feedkeys("\<LeftDrag>", 'xt')
+  call assert_equal(3, &cmdheight)
+
+  call test_setmouse(&lines - 3, 1)
+  call feedkeys("\<LeftMouse>", 'xt')
+  call test_setmouse(&lines - 1, 1)
+  call feedkeys("\<LeftDrag>", 'xt')
+  call assert_equal(1, &cmdheight)
+
+  let &mouse = save_mouse
+  set laststatus&
+endfunc
+
 " vim: shiftwidth=2 sts=2 expandtab