I'm seeing some bad behavior with ActiveTcl 8.6.1 on OS X 10.8.5.
I've got a "messagebox" dialog that pops up a modal dialog. Interacting with the dialog sets a "choice" variable; I do a grab on the dialog, vwait on the "choice" variable, and then return the choice to the caller:
When it hits the "vwait" the dialog appears, garbled, and the app hangs. If I put an [update idletasks] in before the "vwait", it hangs in the [update idletasks]. If I remove both and let it return to the event loop, the dialog appears in the normal way. So the problem appears to have something to do with re-entering the event loop.
This is code I've been using for five or six years now; it works fine on Linux and Windows, and (until now) on OS X as well.
At present, it's happening only in the context of my application; I've not yet been able to create a simple bug script.