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.

GUIFrontend.cc

@@ -0,0 +1,170 @@
+#include <SDL.h>
+#include <SDL_opengl.h>
+
+#include <imgui.h>
+#include <backends/imgui_impl_sdl2.h>
+#include <backends/imgui_impl_opengl3.h>
+#include <cstdio>
+#include <string>
+#include <cstring>
+#include <cstdlib>
+
+#include "Editor.h"
+#include "Command.h"
+#include "GUIFrontend.h"
+#include "Font.h" // embedded default font (DefaultFontRegular)
+
+static const char *kGlslVersion = "#version 150"; // GL 3.2 core (macOS compatible)
+
+bool
+GUIFrontend::Init(Editor &ed)
+{
+	if (SDL_Init(SDL_INIT_VIDEO | SDL_INIT_TIMER) != 0) {
+		return false;
+	}
+
+	// GL attributes for core profile
+	SDL_GL_SetAttribute(SDL_GL_CONTEXT_FLAGS, 0);
+	SDL_GL_SetAttribute(SDL_GL_CONTEXT_PROFILE_MASK, SDL_GL_CONTEXT_PROFILE_CORE);
+	SDL_GL_SetAttribute(SDL_GL_CONTEXT_MAJOR_VERSION, 3);
+	SDL_GL_SetAttribute(SDL_GL_CONTEXT_MINOR_VERSION, 2);
+	SDL_GL_SetAttribute(SDL_GL_DOUBLEBUFFER, 1);
+	SDL_GL_SetAttribute(SDL_GL_DEPTH_SIZE, 24);
+	SDL_GL_SetAttribute(SDL_GL_STENCIL_SIZE, 8);
+
+	window_ = SDL_CreateWindow(
+		"kte",
+		SDL_WINDOWPOS_CENTERED, SDL_WINDOWPOS_CENTERED,
+		width_, height_,
+		SDL_WINDOW_OPENGL | SDL_WINDOW_RESIZABLE | SDL_WINDOW_ALLOW_HIGHDPI);
+	if (!window_)
+		return false;
+
+	gl_ctx_ = SDL_GL_CreateContext(window_);
+	if (!gl_ctx_)
+		return false;
+	SDL_GL_MakeCurrent(window_, gl_ctx_);
+	SDL_GL_SetSwapInterval(1); // vsync
+
+	IMGUI_CHECKVERSION();
+	ImGui::CreateContext();
+ ImGuiIO &io = ImGui::GetIO();
+ (void) io;
+ ImGui::StyleColorsDark();
+
+	if (!ImGui_ImplSDL2_InitForOpenGL(window_, gl_ctx_))
+		return false;
+	if (!ImGui_ImplOpenGL3_Init(kGlslVersion))
+		return false;
+
+	// Initialize editor reported dimensions to pixels for now
+	int w, h;
+	SDL_GetWindowSize(window_, &w, &h);
+	width_  = w;
+	height_ = h;
+    ed.SetDimensions(static_cast<std::size_t>(height_), static_cast<std::size_t>(width_));
+
+    // Initialize GUI font from embedded default
+    LoadGuiFont_(nullptr, 16.f);
+
+    return true;
+}
+
+
+void
+GUIFrontend::Step(Editor &ed, bool &running)
+{
+	SDL_Event e;
+	while (SDL_PollEvent(&e)) {
+		ImGui_ImplSDL2_ProcessEvent(&e);
+		switch (e.type) {
+			case SDL_QUIT:
+				running = false;
+				break;
+			case SDL_WINDOWEVENT:
+				if (e.window.event == SDL_WINDOWEVENT_SIZE_CHANGED) {
+					width_  = e.window.data1;
+					height_ = e.window.data2;
+					ed.SetDimensions(static_cast<std::size_t>(height_),
+					                 static_cast<std::size_t>(width_));
+				}
+				break;
+			default:
+				break;
+		}
+		// Map input to commands
+		input_.ProcessSDLEvent(e);
+	}
+
+	// Execute pending mapped inputs (drain queue)
+	for (;;) {
+		MappedInput mi;
+		if (!input_.Poll(mi))
+			break;
+		if (mi.hasCommand) {
+			Execute(ed, mi.id, mi.arg, mi.count);
+			if (mi.id == CommandId::Quit || mi.id == CommandId::SaveAndQuit) {
+				running = false;
+			}
+		}
+	}
+
+ // Start a new ImGui frame
+ ImGui_ImplOpenGL3_NewFrame();
+ ImGui_ImplSDL2_NewFrame(window_);
+ ImGui::NewFrame();
+
+ // No runtime font UI; always use embedded font.
+
+ // Draw editor UI
+ renderer_.Draw(ed);
+
+	// Render
+	ImGui::Render();
+	int display_w, display_h;
+	SDL_GL_GetDrawableSize(window_, &display_w, &display_h);
+	glViewport(0, 0, display_w, display_h);
+	glClearColor(0.1f, 0.1f, 0.11f, 1.0f);
+	glClear(GL_COLOR_BUFFER_BIT);
+	ImGui_ImplOpenGL3_RenderDrawData(ImGui::GetDrawData());
+	SDL_GL_SwapWindow(window_);
+}
+
+
+void
+GUIFrontend::Shutdown()
+{
+	ImGui_ImplOpenGL3_Shutdown();
+	ImGui_ImplSDL2_Shutdown();
+	ImGui::DestroyContext();
+
+	if (gl_ctx_) {
+		SDL_GL_DeleteContext(gl_ctx_);
+		gl_ctx_ = nullptr;
+	}
+	if (window_) {
+		SDL_DestroyWindow(window_);
+		window_ = nullptr;
+	}
+	SDL_Quit();
+}
+
+
+bool
+GUIFrontend::LoadGuiFont_(const char * /*path*/, float size_px)
+{
+    ImGuiIO &io = ImGui::GetIO();
+    io.Fonts->Clear();
+    ImFont *font = io.Fonts->AddFontFromMemoryCompressedTTF(
+        (void*)DefaultFontRegularCompressedData,
+        (int)DefaultFontRegularCompressedSize,
+        size_px);
+    if (!font) {
+        font = io.Fonts->AddFontDefault();
+    }
+    (void) font;
+    io.Fonts->Build();
+    return true;
+}
+
+// No runtime font reload or system font resolution in this simplified build.