Tooltips still work fine on Windows and given Serhiy's edit, I presume the same is true on Linux.  As far as I remember and have recorded in NEWS.txt, IDLE on 2.7 is unchanged for over a year, since sometime before 2.7.13 was released.  It is too easy to break something and not notice.  The IDLE tests, such as they are for 2.7, run without error.

If you haven't, run IDLE from a terminal to see if any TclErrors or other exceptions get displayed.  The immediately relevant files in idlelib, if you want to play with with debug prints, are CallTips.py and CallTipWindow.py.