Nyquist can now make a selection and not have it overwritten by Effect.

src/commands/SelectCommand.cpp

@@ -61,6 +61,10 @@ bool SelectTimeCommand::Apply(const CommandContext & context){
    if( !bHasT0 && !bHasT1 )
       return true;
 
+   // Count selection as a do-nothing effect.
+   // Used to invalidate cached selection and tracks.
+   Effect::IncEffectCounter();
+
    if( mFromEnd ){
       double TEnd = context.GetProject()->GetTracks()->GetEndTime();
       context.GetProject()->mViewInfo.selectedRegion.setTimes(TEnd - mT0, TEnd - mT1);
@@ -110,6 +114,11 @@ bool SelectTracksCommand::Apply(const CommandContext &context)
 {
    if( !bHasFirstTrack && !bHasLastTrack )
       return true;
+
+   // Count selection as a do-nothing effect.
+   // Used to invalidate cached selection and tracks.
+   Effect::IncEffectCounter();
+
    int index = 0;
    TrackList *tracks = context.GetProject()->GetTracks();
    int last = wxMax( mFirstTrack, mLastTrack );

src/effects/Effect.cpp

@@ -1266,7 +1266,7 @@ bool Effect::DoEffect(wxWindow *parent,
       returnVal = Process();
    }
 
-   if (returnVal)
+   if (returnVal && (mT1 >= mT0 ))
    {
       selectedRegion->setTimes(mT0, mT1);
    }

src/effects/Effect.h

@@ -269,6 +269,8 @@ class AUDACITY_DLL_API Effect /* not final */ : public wxEvtHandler,
                   long style = DefaultMessageBoxStyle,
                   const wxString& titleStr = wxString{});
 
+   static void IncEffectCounter(){ nEffectsDone++;};
+
 //
 // protected virtual methods
 //

src/effects/nyquist/Nyquist.cpp

@@ -859,8 +859,10 @@ _("Selection too long for Nyquist code.\nMaximum allowed selection is %ld sample
    // Has rug been pulled from under us by some effect done within Nyquist??
    if( nEffectsSoFar == nEffectsDone )
       ReplaceProcessedTracks(success);
-   else
+   else{
       ReplaceProcessedTracks(false); // Do not use the results.
+      mT1 = mT0 - 1.0;// And don't use the times either, in resetting the selection  (make them bogus).
+   }
 
    if (!mProjectChanged)
       em.SetSkipStateFlag(true);