Lots of updates:

1. Scrolling and click to set the cursor works.
2. GUI now uses Brass Mono as the font.
3. A lot of stability and other updates.

Buffer.cc

@@ -0,0 +1,142 @@
+#include "Buffer.h"
+
+#include <fstream>
+#include <sstream>
+#include <filesystem>
+
+
+Buffer::Buffer() = default;
+
+
+Buffer::Buffer(const std::string &path)
+{
+	std::string err;
+	OpenFromFile(path, err);
+}
+
+
+bool
+Buffer::OpenFromFile(const std::string &path, std::string &err)
+{
+	// If the file doesn't exist, initialize an empty, non-file-backed buffer
+	// with the provided filename. Do not touch the filesystem until Save/SaveAs.
+	if (!std::filesystem::exists(path)) {
+		rows_.clear();
+		nrows_          = 0;
+		filename_       = path;
+		is_file_backed_ = false;
+		dirty_          = false;
+
+		// Reset cursor/viewport state
+		curx_      = cury_    = rx_ = 0;
+		rowoffs_   = coloffs_ = 0;
+		mark_set_  = false;
+		mark_curx_ = mark_cury_ = 0;
+
+		return true;
+	}
+
+	std::ifstream in(path, std::ios::in | std::ios::binary);
+	if (!in) {
+		err = "Failed to open file: " + path;
+		return false;
+	}
+
+	rows_.clear();
+	std::string line;
+	while (std::getline(in, line)) {
+		// std::getline strips the '\n', keep raw line
+		if (!line.empty() && !in.good()) {
+			// fallthrough
+		}
+		// Handle potential Windows CRLF: strip trailing '\r'
+		if (!line.empty() && line.back() == '\r') {
+			line.pop_back();
+		}
+		rows_.push_back(line);
+	}
+
+	// If file ends with a trailing newline, getline will have produced an empty
+	// last line already. If the file is empty and no lines were read, keep rows_ empty.
+
+	nrows_          = rows_.size();
+	filename_       = path;
+	is_file_backed_ = true;
+	dirty_          = false;
+
+	// Reset cursor/viewport state
+	curx_      = cury_    = rx_ = 0;
+	rowoffs_   = coloffs_ = 0;
+	mark_set_  = false;
+	mark_curx_ = mark_cury_ = 0;
+
+	return true;
+}
+
+
+bool
+Buffer::Save(std::string &err) const
+{
+	if (!is_file_backed_ || filename_.empty()) {
+		err = "Buffer is not file-backed; use SaveAs()";
+		return false;
+	}
+	std::ofstream out(filename_, std::ios::out | std::ios::binary | std::ios::trunc);
+	if (!out) {
+		err = "Failed to open for write: " + filename_;
+		return false;
+	}
+	for (std::size_t i = 0; i < rows_.size(); ++i) {
+		out.write(rows_[i].data(), static_cast<std::streamsize>(rows_[i].size()));
+		if (i + 1 < rows_.size()) {
+			out.put('\n');
+		}
+	}
+	if (!out.good()) {
+		err = "Write error";
+		return false;
+	}
+	// Note: const method cannot change dirty_. Intentionally const to allow UI code
+	// to decide when to flip dirty flag after successful save.
+	return true;
+}
+
+
+bool
+Buffer::SaveAs(const std::string &path, std::string &err)
+{
+	// Write to the given path
+	std::ofstream out(path, std::ios::out | std::ios::binary | std::ios::trunc);
+	if (!out) {
+		err = "Failed to open for write: " + path;
+		return false;
+	}
+	for (std::size_t i = 0; i < rows_.size(); ++i) {
+		out.write(rows_[i].data(), static_cast<std::streamsize>(rows_[i].size()));
+		if (i + 1 < rows_.size()) {
+			out.put('\n');
+		}
+	}
+	if (!out.good()) {
+		err = "Write error";
+		return false;
+	}
+
+	filename_       = path;
+	is_file_backed_ = true;
+	dirty_          = false;
+	return true;
+}
+
+
+std::string
+Buffer::AsString() const
+{
+	std::stringstream ss;
+	ss << "Buffer<" << this->filename_;
+	if (this->Dirty()) {
+		ss << "*";
+	}
+	ss << ">: " << rows_.size() << " lines";
+	return ss.str();
+}