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)

src/mouse.c

@@ -1823,7 +1823,7 @@ retnomove:
 	if (dragwin != NULL)
 	{
 	    // Drag the status line
-	    count = row - dragwin->w_winrow - dragwin->w_height + 1
+	    count = row - W_WINROW(dragwin) - dragwin->w_height + 1
 							     - on_status_line;
 	    win_drag_status_line(dragwin, count);
 	    did_drag |= count;

src/testdir/test_winbar.vim

@@ -161,4 +161,32 @@ func Test_winbar_not_visible_custom_statusline()
   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

src/version.c

@@ -746,6 +746,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    4984,
 /**/
     4983,
 /**/