Backends: GLFW: inhibit error callback when creating cursors because X11 setups may be missing them + comment (#2980)

examples/imgui_impl_glfw.cpp

@@ -15,6 +15,7 @@
 
 // CHANGELOG
 // (minor and older changes stripped away, please see git history for details)
+//  2020-01-17: Inputs: Disable error callback while assigning mouse cursors because some X11 setup don't have them and it generates errors.
 //  2019-12-05: Inputs: Added support for new mouse cursors added in GLFW 3.4+ (resizing cursors, not allowed cursor).
 //  2019-10-18: Misc: Previously installed user callbacks are now restored on shutdown.
 //  2019-07-21: Inputs: Added mapping for ImGuiKey_KeyPadEnter.
@@ -175,6 +176,11 @@ static bool ImGui_ImplGlfw_Init(GLFWwindow* window, bool install_callbacks, Glfw
     io.ImeWindowHandle = (void*)glfwGetWin32Window(g_Window);
 #endif
 
+    // Create mouse cursors
+    // (By design, on X11 cursors are user configurable and some cursors may be missing. When a cursor doesn't exist,
+    // GLFW will emit an error which will often be printed by the app, so we temporarily disable error reporting.
+    // Missing cursors will return NULL and our _UpdateMouseCursor() function will use the Arrow cursor instead.)
+    GLFWerrorfun prev_error_callback = glfwSetErrorCallback(NULL);
     g_MouseCursors[ImGuiMouseCursor_Arrow] = glfwCreateStandardCursor(GLFW_ARROW_CURSOR);
     g_MouseCursors[ImGuiMouseCursor_TextInput] = glfwCreateStandardCursor(GLFW_IBEAM_CURSOR);
     g_MouseCursors[ImGuiMouseCursor_ResizeNS] = glfwCreateStandardCursor(GLFW_VRESIZE_CURSOR);
@@ -191,6 +197,7 @@ static bool ImGui_ImplGlfw_Init(GLFWwindow* window, bool install_callbacks, Glfw
     g_MouseCursors[ImGuiMouseCursor_ResizeNWSE] = glfwCreateStandardCursor(GLFW_ARROW_CURSOR);
     g_MouseCursors[ImGuiMouseCursor_NotAllowed] = glfwCreateStandardCursor(GLFW_ARROW_CURSOR);
 #endif
+    glfwSetErrorCallback(prev_error_callback);
 
     // Chain GLFW callbacks: our callbacks will call the user's previously installed callbacks, if any.
     g_PrevUserCallbackMousebutton = NULL;