Based on the discussion here and in Issue35387, I built and tested macOS installers for 3.7.2rc1+ and 3.6.8rc1+ using the current tip of the Tk core-8-6-branch (https://core.tcl.tk/tk/info/deb6ecc05e4202e3); I continued to build Tcl from the Tcl 8.6.9 release tarball.  As a reminder, for 3.7.2rc1 and 3.6.8rc1, I used the Tk 8.6.9.1 release tarball with Tcl 8.6.9.

With the current Tk tip, the problem described in this issue seems to have been fixed: Tk windows did not turn black while they were being resized.

However, new regressions appeared.  First, the problem described in Issue35387 is now visible, that is, a separate blank window appears when selecting either "About IDLE" or "Preferences" from the application cascade in the application's menu bar at the top of the screen.  (Note that when launching IDLE.app (by double-clicking on the IDLE icon, for example), that menu cascade is normally labeled "IDLE" but when launching IDLE from the command line, for example, with "python -m idledlib" the menu cascade will usually be labeled "Python" instead of "IDLE"; this is normal behavior.)  These days, it is also possible to bring up the IDLE's Preferences window by using the Options -> Configure IDLE menu.  In that case, the spurious blank window does *not* appear.  If I recall correctly, both the About menu item and the Preferences menu items (but not the Options menu items) are special menu items automatically provided by Tk and/or macOS and that IDLE has code to customize, so that may be a good lead.

Second, I happened to notice that, when running a Python program with a syntax error from an IDLE edit window, the syntax error pop-down message appears twice.  A sequence to reproduce:
!. Open a new edit window (cmd-N).
2. Type: x = $1
3. Save to a file (cmd-S).
4. Run the program (F5).
5. The expected "Invalid syntax" panel pulls down on the edit window.
6. Click the OK button in the panel to dismiss.
7. The panel disappears but is immediately replaced by a second and identical "Invalid syntax" panel. (This is new and incorrect behavior).
8. Again click the OK button in the panel to dismiss.  This time the panel goes away and does not reappear.

Third, running Python test_idle causes a segfault.  From the traceback displayed, it appears that the failing test is calling idlelib/configdialog.py so I wouldn't be surprised if this failure is related to the first issue above, i.e. the appearance of the blank window when Preferences is chosen.  To reproduce, from a Terminal command line:

/path/to/python -m test -w -uall -j3 test_idle

where /path/to/python might be /usr/local/bin/python3.6 (or 3.7)

To make it easier to reproduce the problems, I've made test installers built with the Tk core-8-6-branch temporarily available on python.org.  These are similar to but not exactly the same as what is planned for 3.7.2 final and 3.6.8 final.

https://www.python.org/ftp/python/devtest/python-3.7.2rc1+-macosx10.9.pkg
https://www.python.org/ftp/python/devtest/python-3.6.8rc1+-macosx10.9.pkg