This issue is about test_tix failing when run after test_idle, due to the conjunction of two specific causes, one old (tix importing _default_root), one new (IDLE deleting and not restoring _default_root).  The new cause exposed the old.  Fixing either cause would have negated the conjunction, but at Zack's urging, I fixed both.  I even did one extra bit of cleanup in moving the tkinter and os imports to the top where they belong.  In my view, my patch was more than complete and this issue is fixed and should have remained closed and should be reclosed.

Serhiy, when I wrote the tix patch, I was aware that tix remains buggy in that it will fail if a *user* imports tix, removes _default_root, and calls either of the functions that unconditionally access the attribute.  Since test_tix only tests that the tix can be imported and that tix.Tk runs, there may be more bugs.  And more cleanups might be needed.  However, patching tix to work better is a different issue than this one.  If you want, open a new issue, add some tests, and write the patch you outlined.