#5408: Re-implement IUndoSystem::cancel(). The pending operation is n…

…o longer pushed to the stack only to be removed immediately afterwards.

radiantcore/undo/Stack.h

@@ -77,6 +77,12 @@ class UndoStack
 		_pending = std::make_shared<Operation>(command);
 	}
 
+    void cancel()
+    {
+        // discard the pending operation without storing it in the stack
+        _pending.reset();
+    }
+
 	// Finish the current undo operation
 	bool finish(const std::string& command)
 	{

radiantcore/undo/UndoSystem.cpp

@@ -55,12 +55,11 @@ bool UndoSystem::operationStarted() const
 
 void UndoSystem::cancel()
 {
-	// Try to add the last operation as "temp"
-	if (finishUndo("$TEMPORARY"))
-	{
-		// Instantly remove the added operation
-		_undoStack.pop_back();
-	}
+    if (_activeUndoStack != nullptr)
+    {
+        _activeUndoStack->cancel();
+        setActiveUndoStack(nullptr);
+    }
 }
 
 void UndoSystem::finish(const std::string& command)

radiantcore/undo/UndoSystem.h

@@ -55,19 +55,14 @@ class UndoSystem final :
 	~UndoSystem();
 
 	IUndoStateSaver* getStateSaver(IUndoable& undoable) override;
-
 	void releaseStateSaver(IUndoable& undoable) override;
 
 	void start() override;
-
-	bool operationStarted() const override;
-
-	// greebo: This finishes the current operation and
-	// instantly removes it from the stack
 	void cancel() override;
-
 	void finish(const std::string& command) override;
 
+	bool operationStarted() const override;
+
 	void undo() override;
 	void redo() override;