When closing the IDLE's Preferences dialog the IDLE's Console window and/or Editor window (whichever was active before) doesn't show cursor/caret any more. One must click on Desktop or another window then click again in the IDLE's window in order to regain cursor/caret. This problem shows on macOS High Sierra, Python 3.6.N and 3.7.N (and even 3.5.N), with the provided installers (64 bit, 64/32 bit, Tk 8.6.8) and with self-compiled Python(s). After a number of openings IDLE will crash with an @autoreleasepool issue. I've compiled Tcl/Tk and Python with CC=clang CXX=clang++ MACOSX_DEPLOYMENT_TARGET=10.9 (10.10, 10.12 also), Tcl/Tk with and without --enable-threads, always with --enable-64bit --enable-framework and --enable-dtrace, clang 902.0.39.2 (Apple LLVM version 9.1.0), Python was compiled with and without --enable-dtrace and --enable-optimizations, with --enable-framework.