JAWS fix?: Now do the delay of event handling...

... Queue, don't immediately process, TrackList events, and if listeners want
to hold pointers to tracks, let them use weak_ptr or shared_ptr

src/Track.cpp

@@ -857,19 +857,18 @@ void TrackList::RecalcPositions(TrackNodePointer node)
 
 void TrackList::DeletionEvent()
 {
-   wxCommandEvent e(EVT_TRACKLIST_DELETION);
+   auto e = std::make_unique<wxCommandEvent>(EVT_TRACKLIST_DELETION);
-
+   // wxWidgets will own the event object
-   // PRL:  ProcessEvent, not QueueEvent!  Listeners may need their last
+   QueueEvent(e.release());
-   // chance to examine some removed tracks that are about to be destroyed.
-   ProcessEvent(e);
 }
 
 void TrackList::ResizingEvent(TrackNodePointer node)
 {
-   wxCommandEvent e(EVT_TRACKLIST_RESIZING);
+   auto e = std::make_unique<wxCommandEvent>(EVT_TRACKLIST_RESIZING);
    if (!isNull(node))
-      e.SetClientData(node->get());
+      e->SetClientData(node->get());
-   ProcessEvent(e);
+   // wxWidgets will own the event object
+   QueueEvent(e.release());
 }
 
 void TrackList::Permute(const std::vector<TrackNodePointer> &permutation)
@@ -951,8 +950,6 @@ auto TrackList::Replace(Track * t, value_type &&with) -> value_type
       pTrack->SetOwner(this, node);
       RecalcPositions(node);
 
-      // PRL:  Note:  Send the event while t (now in holder) is not yet
-      // destroyed, so pointers to t that listeners may have are not dangling.
       DeletionEvent();
       ResizingEvent(node);
    }
@@ -974,8 +971,6 @@ TrackNodePointer TrackList::Remove(Track *t)
             RecalcPositions(result);
          }
 
-         // PRL:  Note:  Send the event while t (now in holder) is not yet
-         // destroyed, so pointers to t that listeners may have are not dangling.
          DeletionEvent();
       }
    }
@@ -987,9 +982,6 @@ void TrackList::Clear(bool sendEvent)
    ListOfTracks tempList;
    tempList.swap( *this );
    if (sendEvent)
-      // PRL:  Note:  Send the event while tempList is not yet
-      // destroyed, so pointers to tracks that listeners may have are not
-      // dangling.
       DeletionEvent();
 }
 

src/Track.h

@@ -489,9 +489,6 @@ DECLARE_EXPORTED_EVENT_TYPE(AUDACITY_DLL_API, EVT_TRACKLIST_RESIZING, -1);
 
 // Posted when a track has been deleted from a tracklist.
 // Also posted when one track replaces another
-// When a track has been removed, the event is processed before the track
-// is destroyed, so that listeners that stored a pointer to the track can still
-// use it and correctly check whether the list still contains it.
 DECLARE_EXPORTED_EVENT_TYPE(AUDACITY_DLL_API, EVT_TRACKLIST_DELETION, -1);
 
 class TrackList final : public wxEvtHandler, public ListOfTracks

src/TrackPanel.cpp

@@ -2534,6 +2534,16 @@ wxRect TrackPanel::FindTrackRect( const Track * target, bool label )
       target->GetHeight()
    };
 
+   // PRL:  I think the following very old comment misused the term "race
+   // condition" for a bug that happened with only a single thread.  I think the
+   // real problem referred to, was that this function could be reached, via
+   // TrackPanelAx callbacks, during low-level operations while the TrackList
+   // was not in a consistent state.  Therefore GetLinked() did not imply
+   // that GetLink() was not null.
+   // Now the problem is fixed by delaying the handling of events generated
+   // by TrackList.
+
+   // Old comment:
    // The check for a null linked track is necessary because there's
    // a possible race condition between the time the 2 linked tracks
    // are added and when wxAccessible methods are called.  This is