Was able to make the bug occur after lots of fast clicking around, by double-clicking Preview in Amplify dialog.
The key seems to be the call to wxYield. That function is obsolete, but looking at the documentation for its successor, wxApp::Yield():
"Caution should be exercised, however, since yielding may allow the user to perform actions which are not compatible with the current task. Disabling menu items or whole menus during processing can avoid unwanted reentrance of code: see ::wxSafeYield for a better function."
That sounds like exactly the situations where this has occurred. And the documentation for ::wxSafeYieldsays:
"This function is similar to wxYield, except that it disables the user input to all program windows before calling wxYield and re-enables it again afterwards."
Sounds like what we need. So I went through the code and for all the wxYield and wxGetApp()::Yield occurrences where Audacity is doing some drawing or already in the process of responding to a GUI event, I replaced them with ::wxSafeYield. Replaced all the remaining wxYield calls with calls to wxGetApp()::Yield().
Haven't been able to replicate the bug since these changes. Please test.
