In #34055, Raymond said "tool tips no longer display on the newest mac builds".  On idledev, Walter S reported the same using 64-bit python.org "Python 3.7.0 (v3.7.0:1bf9cc5093, Jun 26 2018, 23:26:24) [Clang 6.0 (clang-600.0.57)] on darwin" on multiple machines with 10.13.5.  He also verified that the htest part of 'python3 -m idlelib.calltip_w' fails (no calltip appears when typing '('.

I have doubts that this is a new problem with IDLE.  calltip.py was not touched for 3.7.0.  calltip_w.py was only touched to change return None to return 'break' where appropriate.

This is one of 3 new Mac-specific failures reported after the release of 3.7.0.  The others are #34047, scrolling problems, and #34120, freeze on some dialog closings.  I have to suspect that the problem is with tkinter (_tkinter) and the new tcl/tk 8.6.8.

How to debug (help appreciated with any of these):
1. Run the 3.6.6 32/64 bit binary, compiled against tk 8.5 instead of 8.6, and see if the same problems occur.  Any problems that do are not 8.6 specific.
2. Find minimal failing cases with as little IDLE-specific code as possible.
3. Trace execution of the IDLE code with prints or debugger to find the point of failure.  For this issue, I would start with adding print calls in the function that creates the tip window.