Investigating a user report of IDLE freezing on OS X when mouse clicking on the code completion menu led me to this issue.  The behavior I see varies depending on the Tk variant in use.  The basic test case is:

1. launch IDLE with an IDLE shell window open
2. open an IDLE edit window for a new file
3. in the edit window, press "a" then "TAB"
4. the code completion window opens as expected
5. using the mouse, click on a selection in the code completion list

With two different X11 Tk's (Debian Linux 8.5.x and OS X X11 8.6.x), clicking on the selection causes the code completion window to disappear but the code completion text does not appear in the edit window.  Further, then pressing "Return", "a", and "Tab" to try a new code completion results in the code completion window momentarily appearing then disappearing.  If one uses the mouse to click on and change focus to another IDLE window (e.g. the shell window), then click on and return focus to the edit window, the code completion window starts working again, at least, until using the mouse to click on a selection again.

With the current Cocoa Tk 8.5.x on OS X (the most commonly used Tk there), the behavior is even more confusing to the user.  After mouse-clicking on a code completion menu item, the code completion menu disappears but then the edit window becomes unresponsive to either the keyboard or the mouse until focus is moved, again by clicking on another IDLE window and then returning to the "hung" edit window.

Oddly enough, the older Carbon Tk 8.4.x on OS X (used with 32-bit-only Pythons) seems to handle this all correctly.

I did not try this with a Windows Tk.  I also did not systematically check all current versions of IDLE with all of those Tk variants but spot checking saw similar behavior on current dev (pre-3.5), 3.4.1, and 2.7.x.