Implement naive small string optimization for undo action by using st…

…ructure padding.

See https://sourceforge.net/p/scintilla/feature-requests/1458/,
most action is only single byte, just store them in the padding.

scintilla/src/CellBuffer.cxx

@@ -26,7 +26,6 @@
 #include "VectorISA.h"
 
 #include "Position.h"
-#include "UniqueString.h"
 #include "SplitVector.h"
 #include "Partitioning.h"
 #include "RunStyles.h"
@@ -333,14 +332,20 @@ class LineVector final : public ILineVector {
 };
 
 void Action::Create(ActionType at_, Sci::Position position_, const char *data_, Sci::Position lenData_, bool mayCoalesce_) {
-	position = position_;
 	at = at_;
 	mayCoalesce = mayCoalesce_;
+	position = position_;
 	lenData = lenData_;
-	data = nullptr;
+	char *ptr = nullptr;
 	if (lenData_) {
-		data = UniqueCopy(data_, lenData_);
+		char *text = smallData;
+		if (lenData_ > smallSize) {
+			text = new char[lenData_];
+			ptr = text;
+		}
+		memcpy(text, data_, lenData_);
 	}
+	data.reset(ptr);
 }
 
 void Action::Clear() noexcept {

scintilla/src/CellBuffer.h

@@ -33,9 +33,11 @@ enum class ActionType : uint8_t { insert, remove, start, container };
  * Actions are used to store all the information required to perform one undo/redo step.
  */
 class Action {
+	static constexpr Sci::Position smallSize = sizeof(size_t) - 2;
 public:
 	ActionType at = ActionType::insert;
 	bool mayCoalesce = false;
+	char smallData[smallSize]{};
 	Sci::Position position = 0;
 	Sci::Position lenData = 0;
 	std::unique_ptr<char[]> data;
@@ -44,7 +46,7 @@ class Action {
 	void Create(ActionType at_, Sci::Position position_ = 0, const char *data_ = nullptr, Sci::Position lenData_ = 0, bool mayCoalesce_ = true);
 	void Clear() noexcept;
 	const char *Data() const noexcept {
-		return data.get();
+		return (lenData <= smallSize) ? smallData : data.get();
 	}
 };