It looks like there’s maybe a neater way to get the current frame
under the mouse...
Lisp_Object frame = Qnil;
NSWindow *w = [NSApp windowWithWindowNumber:
[NSWindow windowNumberAtPoint:[NSEvent mouseLocation]
belowWindowWithWindowNumber:0]];
if (w != nil)
XSETFRAME (frame, ((EmacsView *)[w delegate])->emacsframe);