Thanks for the report.  Taking a quick look at it, it appears that the problem was introduced with the most recently releases of ActiveTcl on OS X, 8.5.18 and 8.6.4.  The autocomplete popups seem to work fine with the previous OS X release of ActiveTcl 8.5 (8.5.17) and with a MacPorts +quartz build of Tcl/Tk 8.6.3.  I know that Kevin Walzer added a lot of fixes to the native OS X version of Tk that were first released with 8.6.4 and 8.5.18 and I think there have been additional fixes since then but are not yet in an official Tcl/Tk release.  I'm cc-ing Kevin here; perhaps this will look familiar.

As workarounds, if you have access to ActiveTcl 8.5.17, you could install that.  Or, if you feel up to building your own framework version of 8.5.17, you could try that.  You *could* also remove 8.5.18 and fall back to the Apple-supplied system Tcl/Tk 8.5.9 but I would strongly recommend not doing that as that version has critical problems, most notably the bug that causes Tk to crash (and, thus, crash IDLE with no opportunity for saving work) by typing multi-character compose codes in text boxes (like option-u on US keyboard layouts).

So it should first be established whether this problem still exists with current trunk of 8.5.x (for that's what the OS X installers link with) and, if so, try to get a new version of ActiveTcl 8.5.x released.  It might help to file an issue with ActiveState (https://bugs.activestate.com/index.cgi) and/or on the Tk issue tracker (https://core.tcl.tk/tk/reportlist).  And it would be even better to have a pure Tcl/Tk test case using wish so that Python is not a factor.  Unfortunately, I'm not going to have much time for the immediate future to shepherd that activity.