auto-position gaps when replacing blocks with gaps

app/widget/timelinewidget/timelineundo.h

@@ -1633,10 +1633,16 @@ public:
     block_(block),
     existing_gap_(nullptr),
     existing_merged_gap_(nullptr),
-    our_gap_(nullptr)
+    our_gap_(nullptr),
+    position_command_(nullptr)
   {
   }
 
+  virtual ~TrackReplaceBlockWithGapCommand() override
+  {
+    delete position_command_;
+  }
+
   virtual Project* GetRelevantProject() const override
   {
     return block_->project();
@@ -1691,6 +1697,11 @@ public:
 
         our_gap_->setParent(track_->parent());
         track_->ReplaceBlock(block_, our_gap_);
+
+        if (!position_command_) {
+          position_command_ = new NodeSetPositionAsChildCommand(our_gap_, track_, our_gap_->index(), track_->Blocks().size(), true);
+        }
+        position_command_->redo();
       }
 
     } else {
@@ -1724,6 +1735,8 @@ public:
       track_->ReplaceBlock(our_gap_, block_);
       our_gap_->setParent(&memory_manager_);
 
+      position_command_->undo();
+
     } else if (existing_gap_) {
 
       // If we're here, assume that we extended an existing gap
@@ -1780,6 +1793,8 @@ private:
   bool existing_gap_precedes_;
   GapBlock* our_gap_;
 
+  NodeSetPositionAsChildCommand* position_command_;
+
   QObject memory_manager_;
 
 };