editor_pane: Workaround for empty ImGui context issue on Windows

On Windows, when I've used dedicated thread for ImGui rendering, and
used TLS for GImGui, there's still a bug:

When I move mouse into editor window, plugin crashes. GDB shows that it
crashes when ImGui's GLFW callback function is accessing backend data
via ImGui_ImplGlfw_GetBackendData() (returns current ImGui context).

This is because GLFW queries window events in main thread, not in
drawing thread. Since the two threads have their own GImGui instance,
GImGui will be null in main thread.

@bear24rw (Max Thurn) provided a workaround: register our own glfw
callbacks to ensure the imgui context is set correctly before fowarding
the call to the imgui backend.

This workaround doesn't aim at multithreading, but works well for me!

Reference: ocornut/imgui#3934 (comment)

WARNING: This commit is not suitable for Linux (with X11).

On Linux, it will cause "Forgot to call Render() or EndFrame() at
the end of the previousframe?" error.

CMakeLists.txt

@@ -392,6 +392,7 @@ if (CONFIG_VST_IMGUI)
         src/ImGui/mainwindow.cpp
         src/ImGui/subwindow.cpp
         src/ImGui/editor_pane.cpp
+        src/ImGui/glfw_callbacks.cpp
     )
     target_include_directories (${PROJECT_NAME}_vst_imgui PRIVATE
         ${CMAKE_SOURCE_DIR}/vendor

src/ImGui/editor_pane.cpp

@@ -270,8 +270,19 @@ int ImguiEditor::setupImGui()
     //ImGui::StyleColorsClassic();
 
     // Setup Platform/Renderer backends
+    // NOTICE: On Windows, I need to register my own callbacks to workaround
+    //         with empty context issue.
+    //         See glfw_callbacks.cpp for more details.
+#if _WIN32
+    ImGui_ImplGlfw_InitForOpenGL(window, false); // Do not register callbacks automatically
+    ImGui_ImplOpenGL2_Init();
+
+    // Register my own callbacks
+    _setMyGLFWCallbacks();
+#else // This workaround causes error on Linux
     ImGui_ImplGlfw_InitForOpenGL(window, true);
     ImGui_ImplOpenGL2_Init();
+#endif
 
     /* The font is loaded from generated/font.h. The font file is in generated by the
      * binary_to_source utility included in Dear ImGui, this util is built and run by

src/ImGui/editor_pane.h

@@ -189,6 +189,15 @@ class ImguiEditor
     // Window / panel
     void _AmsynthWindow_Main();
     void _AmsynthWindow_Preset();
+
+    // GLFW Callbacks
+    void _setMyGLFWCallbacks();
+
+    void _charCallback(unsigned int c);
+    void _cursorEnterCallback(int entered);
+    void _mouseButtonCallback(int button, int action, int mods);
+    void _scrollCallback(double xoffset, double yoffset);
+    void _keyCallback(int key, int scancode, int action, int mods);
 };
 
 // The drawing thread function

src/ImGui/glfw_callbacks.cpp

@@ -0,0 +1,114 @@
+/*
+ *  glfw_callbacks.cpp
+ *
+ *  Copyright (c) 2022 AnClark Liu
+ *
+ *  This file is part of amsynth.
+ *
+ *  amsynth is free software; you can redistribute it and/or modify
+ *  it under the terms of the GNU General Public License as published by
+ *  the Free Software Foundation; either version 2 of the License, or
+ *  (at your option) any later version.
+ *
+ *  amsynth is distributed in the hope that it will be useful,
+ *  but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ *  GNU General Public License for more details.
+ *
+ *  You should have received a copy of the GNU General Public License
+ *  along with amsynth.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+/**
+ * Workaround for empty ImGui context issue on Windows
+ *
+ * On Windows, when I've used dedicated thread for ImGui rendering, and
+ * used TLS for GImGui, there's still a bug:
+ *
+ * When I move mouse into editor window, plugin crashes. GDB shows that it
+ * crashes when ImGui's GLFW callback function is accessing backend data
+ * via ImGui_ImplGlfw_GetBackendData() (returns current ImGui context).
+ *
+ * This is because GLFW queries window events in main thread, not in
+ * drawing thread. Since the two threads have their own GImGui instance,
+ * GImGui will be null in main thread.
+ *
+ * @bear24rw (Max Thurn) provided a workaround: register our own glfw
+ * callbacks to ensure the imgui context is set correctly before fowarding
+ * the call to the imgui backend.
+ *
+ * This workaround doesn't aim at multithreading, but works well for me!
+ *
+ * Reference: https://github.com/ocornut/imgui/pull/3934#issuecomment-873213161
+ *
+ * WARNING: On Linux (with X11), This workaround will cause "Forgot to call
+            Render() or EndFrame() at the end of the previous frame?" error.
+ */
+
+#include "editor_pane.h"
+
+void ImguiEditor::_setMyGLFWCallbacks()
+{
+    // Define intermediate callback functions.
+    // Those intermediates will execute callbacks defined in ImguiEditor's
+    // instance.
+    auto char_callback_func = [](GLFWwindow *w, unsigned int c) {
+        static_cast<ImguiEditor *>(glfwGetWindowUserPointer(w))->_charCallback(c);
+    };
+    auto cursor_enter_callback_func = [](GLFWwindow *w, int entered) {
+        static_cast<ImguiEditor *>(glfwGetWindowUserPointer(w))->_cursorEnterCallback(entered);
+    };
+    auto mouse_button_callback_func = [](GLFWwindow *w, int button, int action, int mods) {
+        static_cast<ImguiEditor *>(glfwGetWindowUserPointer(w))->_mouseButtonCallback(button, action, mods);
+    };
+    auto scroll_callback_func = [](GLFWwindow *w, double xoffset, double yoffset) {
+        static_cast<ImguiEditor *>(glfwGetWindowUserPointer(w))->_scrollCallback(xoffset, yoffset);
+    };
+    auto key_callback_func = [](GLFWwindow *w, int key, int scancode, int action, int mods) {
+        static_cast<ImguiEditor *>(glfwGetWindowUserPointer(w))->_keyCallback(key, scancode, action, mods);
+    };
+
+    // Register my own callbacks
+    glfwSetCharCallback(window, char_callback_func);
+    glfwSetCursorEnterCallback(window, cursor_enter_callback_func);
+    glfwSetMouseButtonCallback(window, mouse_button_callback_func);
+    glfwSetScrollCallback(window, scroll_callback_func);
+    glfwSetKeyCallback(window, key_callback_func);
+
+    // Set window user pointer to ImguiEditor's current instance
+    glfwSetWindowUserPointer(window, this);
+}
+
+// ---------- CALLBACKS ----------
+// These callbacks will first set correct ImGui context, then invoke ImGui's own
+// callbacks. This can prevent GLFW from accessing wrong ImGui instance.
+
+void ImguiEditor::_charCallback(unsigned int c)
+{
+    ImGui::SetCurrentContext(this->myImGuiContext);
+    ImGui_ImplGlfw_CharCallback(this->window, c);
+}
+
+void ImguiEditor::_cursorEnterCallback(int entered)
+{
+    ImGui::SetCurrentContext(this->myImGuiContext);
+    ImGui_ImplGlfw_CursorEnterCallback(this->window, entered);
+}
+
+void ImguiEditor::_mouseButtonCallback(int button, int action, int mods)
+{
+    ImGui::SetCurrentContext(this->myImGuiContext);
+    ImGui_ImplGlfw_MouseButtonCallback(this->window, button, action, mods);
+}
+
+void ImguiEditor::_scrollCallback(double xoffset, double yoffset)
+{
+    ImGui::SetCurrentContext(this->myImGuiContext);
+    ImGui_ImplGlfw_ScrollCallback(window, xoffset, yoffset);
+}
+
+void ImguiEditor::_keyCallback(int key, int scancode, int action, int mods)
+{
+    ImGui::SetCurrentContext(this->myImGuiContext);
+    ImGui_ImplGlfw_KeyCallback(this->window, key, scancode, action, mods);
+}