Change procedure to choose append-record tracks once again...

... Allow recording stereo into non-adjacent mono tracks, if they are selected.

Prefer the earliest-starting set of tracks in the track list, among either
selected tracks, or else among all tracks.

Therefore if selection contains a mono, a stereo, and a mono in that order,
the two monos may be chosen to record new stereo.

src/toolbars/ControlToolBar.cpp

@@ -1023,12 +1023,11 @@ void ControlToolBar::OnRecord(wxCommandEvent &evt)
       double allt0 = t0;
 
       using Candidates = std::vector<WaveTrack*>;
-      Candidates candidates, selectedCandidates;
+      Candidates *candidates{};
-      auto addCandidates = [&](Candidates &candidates, WaveTrack *candidate){
+      Candidates greedy, best;
-         if (candidates.size() == recordingChannels)
+      Candidates selectedGreedy, selectedBest;
-            // nothing left to do
+      auto addCandidates = [&](Candidates &greedy, Candidates &best,
-            return;
+                               WaveTrack *candidate){
-
          if (candidate->GetLink() && !candidate->GetLinked())
             return;
 
@@ -1042,18 +1041,25 @@ void ControlToolBar::OnRecord(wxCommandEvent &evt)
                  ? static_cast<WaveTrack*>(channel->GetLink()) : nullptr)
             ++nChannels;
 
-         // Accumulate consecutive single channel tracks, or else one track of
+         // Greedy looks either for a single best-fit or for enough mono
-         // the exact number of channels
+         // channels, even if we choose more
-         if (nChannels > 1)
+         // than one and they are not successive, but this partially accumulated
-            candidates.clear();
+         // result might not become complete
+         if (greedy.size() + nChannels <= recordingChannels &&
+             ( nChannels == 1 || nChannels == recordingChannels )) {
+            for (auto channel = candidate; channel;
+                 channel = channel->GetLinked()
+                    ? static_cast<WaveTrack*>(channel->GetLink()) : nullptr)
+               greedy.push_back(channel);
+         }
 
-         if (nChannels == 1 || // <- comment this out to disallow recording
+         // Best-fit looks for the exact number of channels in one place
-                               // stereo into two adjacent mono tracks
+         if (best.empty() &&
              nChannels == recordingChannels) {
             for (auto channel = candidate; channel;
                  channel = channel->GetLinked()
                     ? static_cast<WaveTrack*>(channel->GetLink()) : nullptr)
-               candidates.push_back(channel);
+               best.push_back(channel);
          }
       };
       if (appendRecord) {
@@ -1068,9 +1074,9 @@ void ControlToolBar::OnRecord(wxCommandEvent &evt)
                if (wt->GetEndTime() > allt0) {
                   allt0 = wt->GetEndTime();
                }
-               addCandidates( candidates, wt );
+               addCandidates( greedy, best, wt );
                if (wt->GetSelected())
-                  addCandidates( selectedCandidates, wt );
+                  addCandidates( selectedGreedy, selectedBest, wt );
                if (wt->GetSelected()) {
                   if (wt->GetEndTime() > t0) {
                      t0 = wt->GetEndTime();
@@ -1079,26 +1085,40 @@ void ControlToolBar::OnRecord(wxCommandEvent &evt)
             }
          }
 
-         // candidate null implies selectedCandidate also null
+         // Which tracks will be appended?
-         if( candidates.empty() )
+         // First, prefer selected to non-selected if we can.  Then,
+         // if enough were found the "greedy" way, and they may begin earlier
+         // in the track list than the "best," prefer them
+
+         if (selectedGreedy.size() == recordingChannels)
+            candidates = &selectedGreedy;
+         else if (!selectedBest.empty())
+            candidates = &selectedBest;
+
+         if (!candidates) {
+//          if (greedy.size() == recordingChannels)
+//             candidates = &greedy;
+//          else
+            if (!best.empty())
+               candidates = &best;
+            if (candidates) {
+               // t0 is now: max(selection-start, end-of-selected-wavetracks)
+               // allt0 is:  max(selection-start, end-of-all-tracks)
+               // Use end time of all wave tracks if none selected
+               t0 = allt0;
+            }
+         }
+
+         if( !candidates )
             appendRecord = false;
       }
 
       if (appendRecord) {
-
-         // t0 is now: max(selection-start, end-of-selected-wavetracks)
-         // allt0 is:  max(selection-start, end-of-all-tracks)
-         // Use end time of all wave tracks if none selected
-         if (selectedCandidates.empty()) {
-            t0 = allt0;
-         }
-
          // Append recording:
          // Pad selected/all wave tracks to make them all the same length
          // Remove recording tracks from the list of tracks for duplex ("overdub")
          // playback.
-         for (auto channel :
+         for (auto channel : *candidates)
-              selectedCandidates.empty() ? candidates : selectedCandidates)
          {
             auto wt = Track::Pointer<WaveTrack>(channel);