The scrollbar control in grip mode should support NCHITTEST better than what's currently happening. It seems Wine has some fixes related to this.
Involved modules: comctl32/themed_scrollbar.c ; user32/controls/scrollbar.c
I attach a test application: test_frame.zip that exercises the behaviour: it creates a simple-border window (it's not a resizing border) together with one sizegrip control:
On Windows 2003 and Windows 10, resizing the window using the sizegrip works, while it doesn't on ReactOS.