In flwm/ fltk 1.10, used in core 4.x and 5.x, the mose auto change its shape in a oblique arrow line when the mouse is on a window corner. But in the new fltk 1.3 the mouse does not show this behavior, so it is VERY hard to grab the corner of any window to resize.
I think this is in both 32 and 64 bits versions, and both normal/top-side window title.