IDLE currently uses tooltips only for calltips describing a function's call signature.  Within Lib/idlelib:
calltip.py (renamed from calltips.py after the 3.7.0/3.6.6 release) has the logic for when to raise one and its content.
calltip_w.py (which I intend to merge into calltip.py) has the code for the calltip itself and its closing.

Issue #1529353 and PR7626) proposes a second use of tooltips. I would also like to add helptips to some dialog fields and if possible, some menu entries.

tooltip.py has simple generic tooltip code.  calltip_w does not import this but says in its docstring 'after tooltip.py'. Since tooltip is not currently used anywhere in IDLE, I have considered removing it.

Issue #33839 and PR7683 instead propose to upgrade tooltip and refactor calltip_w to import and use tooltip.  I have held off merging the PR because of tooltips not working on Mac.  But I need to do so soon to unblock the issue above and possible other enchancements.

Both tooltip and calltip_w have run-when-main test functions that work on Windows (and I presume Linux) but fail on 10.13.6 with 3.7.0.  The tooltip test is simpler in that it does not involve any truly IDLE-specific code.  The calltip_w test does not involve calltip.  Both use IDLE's human-verification test driver that provides a root window with test instructions.  In the bash terminal, run
python3 -m idlelib.tooltip
python3 -m idlelib.calltip_w

Kevin, I would be very appreciative if you can suggest a tweak that makes tooltips work everywhere.