On idledev thread "Mac IDLE 3.7.0 freezes when accessing Preferences", Walter Schnyder reported something similar.  Combining two posts:

"using 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 [64-bit] and came across these [new] bugs on several Macs, all running OS X v 10.13.5.
[Has run IDLE before, back to 3.4.]

Bug A: 
1. From the shell select Preferences in the IDLE menu.
2. Cancel the Preferences to get back to the Shell
3. The shell is frozen. It doesn’t even react to CONTROL-D.

The only way I found to resurrect it is to control-click the shell window and select “Go to file/line” causing an error message to appear. Click OK and the shell comes back to life.

Note: This doesn’t only happen when accessing preferences. It also happens when selecting “About IDLE” in the IDLE menu."

After starting with 'python3 -m idlelib' 

"The same problems occur. There is no error in the terminal window.
But I learned something new: after running the steps for bug A, IDLE is frozen (no blinking cursor, not responding to Control+D. However, if I bring another application’s window to the front (by clicking on it) then click back on IDLE’s window, this resurrects IDLE, the cursor blinks and Control+D quits."

test_idle passes.  When running 'python3 -m idlelib.configdialog' or 'idlelib.help_about', which run a human-viewed test, there is no problem when closing either dialog.

Vlad, if I understand your post, you *do* see the problem with 3.5? With the python.org installer and tk 8.5? As far as I know, no one has had a problem with this.  Or only with a private compile with tk 8.6?

This is important because 3.5 IDLE has not been touched since about 18 months, and if there is only a problem when using tk 8.6, then I have to suspect that this and other new problems are result of tk 8.6 on Mac or tkinter needing a patch to support it.