IDLE tests passed the Azure macOS test (when it does not timeout) before and since #17535 was merged.  So this failure report initially surprised me.  But then I checked Display Build Info on
https://dev.azure.com/Python/cpython/_build/results?buildId=47634&view=logs&j=18d1a34d-6940-5fc1-f55b-405e2fba32b1

tkinter.TCL_VERSION: 8.5
tkinter.TK_VERSION: 8.5
tkinter.info_patchlevel: 8.5.9

Oh dear. Azure is testing Tkinter, Turtle, and IDLE with Apple's ancient, buggy, and somewhat obsolete system tcl/tk, which we do not support. 

Steve, for years we have closed issues reporting failures and crashes with 8.5.9 and told users to install something more recent.
https://www.python.org/download/mac/tcltk/
We now directly supply something current in the Mac installer.  We need tkinter, etc, tested on macOS with the tcl/tk that we support and supply, just like Azure does on Windows.  

tkinter.TCL_VERSION: 8.6
tkinter.TK_VERSION: 8.6
tkinter.info_patchlevel: 8.6.9

Ned, for the present, we should skip the tests on Mac. If the code and test were made to work on 8.6.x, they might well fail on 8.5.9.  PR 15010.  You will have to un-comment the skips to test for real.

What behavior do users see.  Can you check?  The test says that clicking on line 2 selects line 1.  Does it?  That would be awful. What does click and drag do?  Given the off-by-one reports, I made the text five lines so that a 3-line drag should have unselected lines above and below.  I added a test for drag up as well as down.