CellularPanel saves and restores focus for each click-drag-release...

... while the project makes note of which window has temporarily given up the
focus; and TrackPanel can detect that, so that the yellow border of the focused
track is still drawn while the panel "lends" the focus.

Why do all this?  So that, when the ruler is another CellularPanel, ESC key
will work to abort drags in the ruler, but TrackPanel appearance won't change
during the drags.

src/CellularPanel.cpp

@@ -106,6 +106,10 @@ void CellularPanel::Uncapture(wxMouseState *pState)
    if (HasCapture())
       ReleaseMouse();
    HandleMotion( *pState );
+
+   auto lender = GetProject()->mFocusLender.get();
+   if (lender)
+      lender->SetFocus();
 }
 
 bool CellularPanel::CancelDragging()
@@ -654,10 +658,6 @@ try
       GetParent()->GetEventHandler()->ProcessEvent(e);
    }
 
-   if (event.ButtonDown()) {
-      SetFocus();
-   }
-
    if (event.Leaving())
    {
       if ( !state.mUIHandle )
@@ -775,6 +775,9 @@ void CellularPanel::HandleClick( const TrackPanelMouseEvent &tpmEvent )
       if (refreshResult & RefreshCode::Cancelled)
          state.mUIHandle.reset(), handle.reset(), ClearTargets();
       else {
+         if( !HasFocus() )
+            SetFocus();
+
          state.mpClickedCell = pCell;
 
          // Perhaps the clicked handle wants to update cursor and state message
@@ -812,13 +815,20 @@ void CellularPanel::DoContextMenu( TrackPanelCell *pCell )
    ProcessUIHandleResult(pCell, pCell, refreshResult);
 }
 
-void CellularPanel::OnSetFocus(wxFocusEvent & WXUNUSED(event))
+void CellularPanel::OnSetFocus(wxFocusEvent &event)
 {
+   auto &ptr = GetProject()->mFocusLender;
+   if ( !ptr )
+      ptr = event.GetWindow();
+   
    SetFocusedCell();
 }
 
 void CellularPanel::OnKillFocus(wxFocusEvent & WXUNUSED(event))
 {
+   // Forget any borrowing of focus
+   GetProject()->mFocusLender = NULL;
+
    if (AudacityProject::HasKeyboardCapture(this))
    {
       AudacityProject::ReleaseKeyboard(this);

src/Project.cpp

@@ -6258,3 +6258,8 @@ void AudacityProject::PlaybackScroller::OnTimer(wxCommandEvent &event)
       trackPanel->Refresh(false);
    }
 }
+
+bool AudacityProject::IsFocused( const wxWindow *window ) const
+{
+   return window == mFocusLender || window == wxWindow::FindFocus();
+}

src/Project.h

@@ -834,6 +834,12 @@ public:
    std::shared_ptr<BackgroundCell> GetBackgroundCell() const
       { return mBackgroundCell; }
 
+   wxWindowRef mFocusLender;
+
+   // Return true if the window is really focused, or if focus was borrowed
+   // from it
+   bool IsFocused( const wxWindow *window ) const;
+
    DECLARE_EVENT_TABLE()
 };
 

src/TrackPanel.cpp

@@ -1270,7 +1270,7 @@ void TrackPanel::DrawEverythingElse(TrackPanelDrawingContext &context,
    //     if (GetFocusedTrack() != NULL) {
    // the highlight was reportedly drawn even when something else
    // was the focus and no highlight should be drawn. -RBD
-   if (GetFocusedTrack() != NULL && wxWindow::FindFocus() == this) {
+   if (GetFocusedTrack() != NULL && GetProject()->IsFocused( this )) {
       HighlightFocusedTrack(dc, focusRect);
    }