Part of the problem of debugging this in the past is that there was never really a proper table made to aid in debugging. There are 3 variables at play:

1. PySimpleGUI version
2. tkinter version
3. MacOS version

Much of the time, it's been the tkinter version that's been the focus. I'm unsure if tkinter is responsible for all of the problems seen.

Here is my breakdown:

PySimpleGUI v4.30.0
tkinter v8.6
MacOS 10.15.7

I also want to mention that I use Homebrew for my python binaries.

Edit:

Python v3.8.1

macOS High Sierra Version 10.13.6
PySimpleGUI Version 4.30.0.12
tcl ver = 8.6 tkinter version = 8.6
tcl detailed version = 8.6.10
Python Version 3.8.6

And... it works!

edit: The environment that gave me the fewest headaches regarding tkinter:

Homebrew -> brew install tcl-tk
Homebrew -> brew install pyenv -> Python (see below)

env \
  PATH="$(brew --prefix tcl-tk)/bin:$PATH" \
  LDFLAGS="-L$(brew --prefix tcl-tk)/lib" \
  CPPFLAGS="-I$(brew --prefix tcl-tk)/include" \
  PKG_CONFIG_PATH="$(brew --prefix tcl-tk)/lib/pkgconfig" \
  CFLAGS="-I$(brew --prefix tcl-tk)/include" \
  PYTHON_CONFIGURE_OPTS="--enable-framework --with-tcltk-includes='-I$(brew --prefix tcl-tk)/include' --with-tcltk-libs='-L$(brew --prefix tcl-tk)/lib -ltcl8.6 -ltk8.6'" \
  pyenv install 3.8.6

pyenv global 3.8.6

and to make this my default python:

echo -e 'if command -v pyenv 1>/dev/null 2>&1; then\n eval "$(pyenv init -)"\nfi' >> ~/.bash_profile

I followed all the steps listed by @norambna and I got it to work for me. It was a bit cumbersome to jump through all the hoops though.

Python v3.8.6
PySimpleGUI v4.30.0
tkinter v8.6
MacOS 10.15.7
pyenv v1.2.21
tcl-tk v8.6.10

I'm glad it helped! I know I had a hard time till finding a solution to all my macOS/tkinter problems. This won't solve all tkinter issues, but it's clearly the best way to work with it on macOS.

Does it appear that tkinter version 8.6.10 is what fixes this problem?

If all versions have problems except 8.6.10, then I could check for that version and apply the "fix" only to non-8.6.10 releases.

What I've been really confused about is if this is a recent problem or has no-titlebar never worked on the Mac. It seems odd that it would have never worked, but it's difficult to tell for sure.

The "fix" I've seen is potentially going to break other stuff which is why I've hesitated adding it to the code.

These steps are pretty ugly so I think that I've got to do something about this. Users can't be expected to go through this much trouble. I'll pull together a release for GitHub with the fix that checks the tkinter version number and only applies it when the version is not 8.6.10.

Does it appear that tkinter version 8.6.10 is what fixes this problem?

For me, yes. 8.6.10 solves a ton of issues.

I just posted 4.30.0.13 that contains a patch if the version is less than 8.6.10.

If this "fixes" the problem, then I want to try one more patch. This new patch clears the wm_overrideredirect flag after first setting it.

root.wm_overrideredirect(True)
root.wm_overrideredirect(False)

What I would like to try next is to set it to True again so that the operation is a True False True. The idea here is that for some systems, it may be working correctly if the value is set to True. What I don't want to do is break all of the installs where it is functioning correctly today and setting it to True again may provide a workaround that does the right thing on all versions of tkinter on Mac.

You will see this message printed on the console if the patch is applied:
* Applying Mac no_titlebar patch *

One way to test this is through the test harness which will save you from having to write code to call and test popups. One of the tabs in the test harness (invoked by running PySimpleGUI.py file or calling sg.main()) is used to test popups.

This will let you quickly check both the no-titlebar window as well as by calling popups with a titlebar.

It may be difficult to spot because the background is the same as your test window. If you click No Titlebar button, you'll see this little window

I hope this fixes the non 8.6.10 version as well as not running on 8.6.10+. If it doesn't I'll quickly put together another test to see if setting it back again works.

If we get this nailed down, I'll get the release posted to PyPI as a high-priority release.