apprt/gtk: fix SIGSEGV on ImGui GLArea re-realize

pkg/dcimgui/ext.cpp

@@ -27,4 +27,25 @@ CIMGUI_API void ImGuiStyle_ImGuiStyle(cimgui::ImGuiStyle* self)
     ::ImGuiStyle defaults;
     *reinterpret_cast<::ImGuiStyle*>(self) = defaults;
 }
+
+// Perform the OpenGL3 backend shutdown and then zero out the imgl3w
+// function pointer table. ImGui_ImplOpenGL3_Shutdown() calls
+// imgl3wShutdown() which dlcloses the GL library handles but does not
+// zero out the function pointers. A subsequent ImGui_ImplOpenGL3_Init()
+// sees the stale (non-null) pointers, skips loader re-initialization,
+// and crashes when calling through them. Zeroing the table forces the
+// next Init to reload the GL function pointers via imgl3wInit().
+#ifndef IMGUI_DISABLE
+#if __has_include("backends/imgui_impl_opengl3.h")
+#include "backends/imgui_impl_opengl3.h"
+#include "backends/imgui_impl_opengl3_loader.h"
+
+CIMGUI_API void ImGui_ImplOpenGL3_ShutdownWithLoaderCleanup()
+{
+    ::ImGui_ImplOpenGL3_Shutdown();
+    memset(&imgl3wProcs, 0, sizeof(imgl3wProcs));
+}
+#endif
+#endif
+
 }

pkg/dcimgui/main.zig

@@ -16,6 +16,10 @@ pub extern fn ImGui_ImplOpenGL3_Shutdown() callconv(.c) void;
 pub extern fn ImGui_ImplOpenGL3_NewFrame() callconv(.c) void;
 pub extern fn ImGui_ImplOpenGL3_RenderDrawData(draw_data: *c.ImDrawData) callconv(.c) void;
 
+// Extension: shutdown the OpenGL3 backend and zero out the imgl3w function
+// pointer table so a subsequent Init can re-initialize the loader.
+pub extern fn ImGui_ImplOpenGL3_ShutdownWithLoaderCleanup() callconv(.c) void;
+
 // Metal backend
 pub extern fn ImGui_ImplMetal_Init(device: *anyopaque) callconv(.c) bool;
 pub extern fn ImGui_ImplMetal_Shutdown() callconv(.c) void;

src/apprt/gtk/class/imgui_widget.zig

@@ -257,8 +257,18 @@ pub const ImguiWidget = extern struct {
             priv.tick_callback_id = 0;
         }
 
+        // Unrealize is not guaranteed to be called with a current GL context,
+        // so we make it current for ImGui cleanup.
+        priv.gl_area.makeCurrent();
+        if (priv.gl_area.getError()) |err| {
+            log.warn("GLArea for Dear ImGui widget failed to realize: {s}", .{err.f_message orelse "(unknown)"});
+            return;
+        }
+
         self.setCurrentContext() catch return;
-        cimgui.ImGui_ImplOpenGL3_Shutdown();
+        cimgui.ImGui_ImplOpenGL3_ShutdownWithLoaderCleanup();
+        cimgui.c.ImGui_DestroyContext(priv.ig_context);
+        priv.ig_context = null;
     }
 
     /// Handle a request to resize the GLArea