Small window flickering when move out the edge of main window.

[JTtNinjaCode]

Version/Branch of Dear ImGui:

Version 1.90.6, Branch: docking (master/docking/etc.)

Back-ends:

imgui_impl_opengl3.cpp + imgui_impl_glfw.cpp

Compiler, OS:

Windows 10 + MSVC 2022

Full config/build information:

imgui config on the computer in problem.

Dear ImGui 1.90.6 (19060)
--------------------------------
sizeof(size_t): 8, sizeof(ImDrawIdx): 2, sizeof(ImDrawVert): 20
define: __cplusplus=199711
define: _WIN32
define: _WIN64
define: _MSC_VER=1938
define: _MSVC_LANG=201402
define: IMGUI_HAS_VIEWPORT
define: IMGUI_HAS_DOCK
--------------------------------
io.BackendPlatformName: imgui_impl_glfw
io.BackendRendererName: imgui_impl_opengl3
io.ConfigFlags: 0x00000443
 NavEnableKeyboard
 NavEnableGamepad
 DockingEnable
 ViewportsEnable
io.ConfigViewportsNoDecoration
io.ConfigInputTextCursorBlink
io.ConfigWindowsResizeFromEdges
io.ConfigMemoryCompactTimer = 60.0
io.BackendFlags: 0x00001C0E
 HasMouseCursors
 HasSetMousePos
 PlatformHasViewports
 HasMouseHoveredViewport
 RendererHasVtxOffset
 RendererHasViewports
--------------------------------
io.Fonts: 1 fonts, Flags: 0x00000000, TexSize: 512,64
io.DisplaySize: 1920.00,1017.00
io.DisplayFramebufferScale: 1.00,1.00
--------------------------------
style.WindowPadding: 8.00,8.00
style.WindowBorderSize: 1.00
style.FramePadding: 4.00,3.00
style.FrameRounding: 0.00
style.FrameBorderSize: 0.00
style.ItemSpacing: 8.00,4.00
style.ItemInnerSpacing: 4.00,4.00

imgui config on the computer without problem.

Dear ImGui 1.90.6 (19060)
--------------------------------
sizeof(size_t): 8, sizeof(ImDrawIdx): 2, sizeof(ImDrawVert): 20
define: __cplusplus=199711
define: _WIN32
define: _WIN64
define: _MSC_VER=1939
define: _MSVC_LANG=201402
define: IMGUI_HAS_VIEWPORT
define: IMGUI_HAS_DOCK
--------------------------------
io.BackendPlatformName: imgui_impl_glfw
io.BackendRendererName: imgui_impl_opengl3
io.ConfigFlags: 0x00000443
 NavEnableKeyboard
 NavEnableGamepad
 DockingEnable
 ViewportsEnable
io.ConfigViewportsNoDecoration
io.ConfigInputTextCursorBlink
io.ConfigWindowsResizeFromEdges
io.ConfigMemoryCompactTimer = 60.0
io.BackendFlags: 0x00001C0E
 HasMouseCursors
 HasSetMousePos
 PlatformHasViewports
 HasMouseHoveredViewport
 RendererHasVtxOffset
 RendererHasViewports
--------------------------------
io.Fonts: 1 fonts, Flags: 0x00000000, TexSize: 512,64
io.DisplaySize: 1280.00,720.00
io.DisplayFramebufferScale: 1.00,1.00
--------------------------------
style.WindowPadding: 8.00,8.00
style.WindowBorderSize: 1.00
style.FramePadding: 4.00,3.00
style.FrameRounding: 0.00
style.FrameBorderSize: 0.00
style.ItemSpacing: 8.00,4.00
style.ItemInnerSpacing: 4.00,4.00

Details:

When I move the small window out of the main window, flickering occurs when it reaches the edge. This is a screenshot of one of the frames.

In my other computer（same as Windows 10 and MSVC 2022）this problem not occur.

The reason I encountered this issue is because I initially wanted to leverage the cross-platform capabilities of GLFW + OpenGL 3, using CMake to quickly set up an ImGui template on various platforms. Below is the link：
https://github.com/JTtNinjaCode/Dear_Imgui_Template.git

Screenshots/Video:

No response

Minimal, Complete and Verifiable Example code:

Code, modify from you example code, but use docking branch, add viewport and docking feature.
Add these code into example code with opengl3 and glfw as backends：

io.ConfigFlags |= ImGuiConfigFlags_DockingEnable;         // Enable Docking
io.ConfigFlags |= ImGuiConfigFlags_ViewportsEnable;       // Enable Viewport

// in main loop
if (io.ConfigFlags & ImGuiConfigFlags_ViewportsEnable) {
  ImGui::UpdatePlatformWindows();
  ImGui::RenderPlatformWindowsDefault();
}

Full Code：

// Dear ImGui: standalone example application for GLFW + OpenGL 3, using programmable pipeline
// (GLFW is a cross-platform general purpose library for handling windows, inputs, OpenGL/Vulkan/Metal graphics context creation, etc.)

// Learn about Dear ImGui:
// - FAQ                  https://dearimgui.com/faq
// - Getting Started      https://dearimgui.com/getting-started
// - Documentation        https://dearimgui.com/docs (same as your local docs/ folder).
// - Introduction, links and more at the top of imgui.cpp

#include "imgui.h"
#include "backends/imgui_impl_glfw.h"
#include "backends/imgui_impl_opengl3.h"
#include <stdio.h>
#define GL_SILENCE_DEPRECATION
#if defined(IMGUI_IMPL_OPENGL_ES2)
#include <GLES2/gl2.h>
#endif
#include <GLFW/glfw3.h> // Will drag system OpenGL headers

// [Win32] Our example includes a copy of glfw3.lib pre-compiled with VS2010 to maximize ease of testing and compatibility with old VS compilers.
// To link with VS2010-era libraries, VS2015+ requires linking with legacy_stdio_definitions.lib, which we do using this pragma.
// Your own project should not be affected, as you are likely to link with a newer binary of GLFW that is adequate for your version of Visual Studio.
#if defined(_MSC_VER) && (_MSC_VER >= 1900) && !defined(IMGUI_DISABLE_WIN32_FUNCTIONS)
#pragma comment(lib, "legacy_stdio_definitions")
#endif

// This example can also compile and run with Emscripten! See 'Makefile.emscripten' for details.
#ifdef __EMSCRIPTEN__
#include "../libs/emscripten/emscripten_mainloop_stub.h"
#endif

static void glfw_error_callback(int error, const char* description)
{
    fprintf(stderr, "GLFW Error %d: %s\n", error, description);
}

// Main code
int main(int, char**)
{
    glfwSetErrorCallback(glfw_error_callback);
    if (!glfwInit())
        return 1;

    // Decide GL+GLSL versions
#if defined(IMGUI_IMPL_OPENGL_ES2)
    // GL ES 2.0 + GLSL 100
    const char* glsl_version = "#version 100";
    glfwWindowHint(GLFW_CONTEXT_VERSION_MAJOR, 2);
    glfwWindowHint(GLFW_CONTEXT_VERSION_MINOR, 0);
    glfwWindowHint(GLFW_CLIENT_API, GLFW_OPENGL_ES_API);
#elif defined(__APPLE__)
    // GL 3.2 + GLSL 150
    const char* glsl_version = "#version 150";
    glfwWindowHint(GLFW_CONTEXT_VERSION_MAJOR, 3);
    glfwWindowHint(GLFW_CONTEXT_VERSION_MINOR, 2);
    glfwWindowHint(GLFW_OPENGL_PROFILE, GLFW_OPENGL_CORE_PROFILE);  // 3.2+ only
    glfwWindowHint(GLFW_OPENGL_FORWARD_COMPAT, GL_TRUE);            // Required on Mac
#else
    // GL 3.0 + GLSL 130
    const char* glsl_version = "#version 130";
    glfwWindowHint(GLFW_CONTEXT_VERSION_MAJOR, 3);
    glfwWindowHint(GLFW_CONTEXT_VERSION_MINOR, 0);
    //glfwWindowHint(GLFW_OPENGL_PROFILE, GLFW_OPENGL_CORE_PROFILE);  // 3.2+ only
    //glfwWindowHint(GLFW_OPENGL_FORWARD_COMPAT, GL_TRUE);            // 3.0+ only
#endif

    // Create window with graphics context
    GLFWwindow* window = glfwCreateWindow(1280, 720, "Dear ImGui GLFW+OpenGL3 example", nullptr, nullptr);
    if (window == nullptr)
        return 1;
    glfwMakeContextCurrent(window);
    glfwSwapInterval(1); // Enable vsync

    // Setup Dear ImGui context
    IMGUI_CHECKVERSION();
    ImGui::CreateContext();
    ImGuiIO& io = ImGui::GetIO(); (void)io;
    io.ConfigFlags |= ImGuiConfigFlags_NavEnableKeyboard;     // Enable Keyboard Controls
    io.ConfigFlags |= ImGuiConfigFlags_NavEnableGamepad;      // Enable Gamepad Controls
    io.ConfigFlags |= ImGuiConfigFlags_DockingEnable;         // Enable Docking
    io.ConfigFlags |= ImGuiConfigFlags_ViewportsEnable;       // Enable Viewport
    // Setup Dear ImGui style
    ImGui::StyleColorsDark();
    //ImGui::StyleColorsLight();

    // Setup Platform/Renderer backends
    ImGui_ImplGlfw_InitForOpenGL(window, true);
#ifdef __EMSCRIPTEN__
    ImGui_ImplGlfw_InstallEmscriptenCanvasResizeCallback("#canvas");
#endif
    ImGui_ImplOpenGL3_Init(glsl_version);

    // Load Fonts
    // - If no fonts are loaded, dear imgui will use the default font. You can also load multiple fonts and use ImGui::PushFont()/PopFont() to select them.
    // - AddFontFromFileTTF() will return the ImFont* so you can store it if you need to select the font among multiple.
    // - If the file cannot be loaded, the function will return a nullptr. Please handle those errors in your application (e.g. use an assertion, or display an error and quit).
    // - The fonts will be rasterized at a given size (w/ oversampling) and stored into a texture when calling ImFontAtlas::Build()/GetTexDataAsXXXX(), which ImGui_ImplXXXX_NewFrame below will call.
    // - Use '#define IMGUI_ENABLE_FREETYPE' in your imconfig file to use Freetype for higher quality font rendering.
    // - Read 'docs/FONTS.md' for more instructions and details.
    // - Remember that in C/C++ if you want to include a backslash \ in a string literal you need to write a double backslash \\ !
    // - Our Emscripten build process allows embedding fonts to be accessible at runtime from the "fonts/" folder. See Makefile.emscripten for details.
    //io.Fonts->AddFontDefault();
    //io.Fonts->AddFontFromFileTTF("c:\\Windows\\Fonts\\segoeui.ttf", 18.0f);
    //io.Fonts->AddFontFromFileTTF("../../misc/fonts/DroidSans.ttf", 16.0f);
    //io.Fonts->AddFontFromFileTTF("../../misc/fonts/Roboto-Medium.ttf", 16.0f);
    //io.Fonts->AddFontFromFileTTF("../../misc/fonts/Cousine-Regular.ttf", 15.0f);
    //ImFont* font = io.Fonts->AddFontFromFileTTF("c:\\Windows\\Fonts\\ArialUni.ttf", 18.0f, nullptr, io.Fonts->GetGlyphRangesJapanese());
    //IM_ASSERT(font != nullptr);

    // Our state
    bool show_demo_window = true;
    bool show_another_window = false;
    ImVec4 clear_color = ImVec4(0.45f, 0.55f, 0.60f, 1.00f);

    // Main loop
#ifdef __EMSCRIPTEN__
    // For an Emscripten build we are disabling file-system access, so let's not attempt to do a fopen() of the imgui.ini file.
    // You may manually call LoadIniSettingsFromMemory() to load settings from your own storage.
    io.IniFilename = nullptr;
    EMSCRIPTEN_MAINLOOP_BEGIN
#else
    while (!glfwWindowShouldClose(window))
#endif
    {
        // Poll and handle events (inputs, window resize, etc.)
        // You can read the io.WantCaptureMouse, io.WantCaptureKeyboard flags to tell if dear imgui wants to use your inputs.
        // - When io.WantCaptureMouse is true, do not dispatch mouse input data to your main application, or clear/overwrite your copy of the mouse data.
        // - When io.WantCaptureKeyboard is true, do not dispatch keyboard input data to your main application, or clear/overwrite your copy of the keyboard data.
        // Generally you may always pass all inputs to dear imgui, and hide them from your application based on those two flags.
        glfwPollEvents();

        // Start the Dear ImGui frame
        ImGui_ImplOpenGL3_NewFrame();
        ImGui_ImplGlfw_NewFrame();
        ImGui::NewFrame();

        // 1. Show the big demo window (Most of the sample code is in ImGui::ShowDemoWindow()! You can browse its code to learn more about Dear ImGui!).
        if (show_demo_window)
            ImGui::ShowDemoWindow(&show_demo_window);

        // 2. Show a simple window that we create ourselves. We use a Begin/End pair to create a named window.
        {
            static float f = 0.0f;
            static int counter = 0;
            printf("%s", ImGui::GetVersion());

            ImGui::Begin("Hello, world!");                          // Create a window called "Hello, world!" and append into it.

            ImGui::Text("This is some useful text.");               // Display some text (you can use a format strings too)
            ImGui::Checkbox("Demo Window", &show_demo_window);      // Edit bools storing our window open/close state
            ImGui::Checkbox("Another Window", &show_another_window);

            ImGui::SliderFloat("float", &f, 0.0f, 1.0f);            // Edit 1 float using a slider from 0.0f to 1.0f
            ImGui::ColorEdit3("clear color", (float*)&clear_color); // Edit 3 floats representing a color

            if (ImGui::Button("Button"))                            // Buttons return true when clicked (most widgets return true when edited/activated)
                counter++;
            ImGui::SameLine();
            ImGui::Text("counter = %d", counter);

            ImGui::Text("Application average %.3f ms/frame (%.1f FPS)", 1000.0f / io.Framerate, io.Framerate);
            ImGui::End();
        }

        // 3. Show another simple window.

        {
            ImGui::Begin("Another Window", &show_another_window);   // Pass a pointer to our bool variable (the window will have a closing button that will clear the bool when clicked)
            ImGui::Text("Hello from another window!");
            if (ImGui::Button("Close Me"))
                show_another_window = false;
            ImGui::End();
        }

        // Rendering
        ImGui::Render();
        int display_w, display_h;
        glfwGetFramebufferSize(window, &display_w, &display_h);
        glViewport(0, 0, display_w, display_h);
        glClearColor(clear_color.x * clear_color.w, clear_color.y * clear_color.w, clear_color.z * clear_color.w, clear_color.w);
        glClear(GL_COLOR_BUFFER_BIT);
        ImGui_ImplOpenGL3_RenderDrawData(ImGui::GetDrawData());

        if (io.ConfigFlags & ImGuiConfigFlags_ViewportsEnable) {
          ImGui::UpdatePlatformWindows();
          ImGui::RenderPlatformWindowsDefault();
        }

        glfwSwapBuffers(window);
    }
#ifdef __EMSCRIPTEN__
    EMSCRIPTEN_MAINLOOP_END;
#endif

    // Cleanup
    ImGui_ImplOpenGL3_Shutdown();
    ImGui_ImplGlfw_Shutdown();
    ImGui::DestroyContext();

    glfwDestroyWindow(window);
    glfwTerminate();

    return 0;
}

[ocornut]

Can you clarify what you mean by flickering by capturing a video of it?

[JTtNinjaCode]

https://youtu.be/FnY64d0JtK8?si=89DVGQ4OHircR3Z4

[ocornut]

It seems to be because RenderPlatformWindowsDefault() changes the current GL context:

if (io.ConfigFlags & ImGuiConfigFlags_ViewportsEnable) 
{
   ImGui::UpdatePlatformWindows();
   ImGui::RenderPlatformWindowsDefault();
}

And immediately after you are swapping the current GL context which is the wrong context:

glfwSwapBuffers(window);

If you look at our example, we are doing:

// Update and Render additional Platform Windows
// (Platform functions may change the current OpenGL context, so we save/restore it to make it easier to paste this code elsewhere.
//  For this specific demo app we could also call glfwMakeContextCurrent(window) directly)
if (io.ConfigFlags & ImGuiConfigFlags_ViewportsEnable)
{
    GLFWwindow* backup_current_context = glfwGetCurrentContext();
    ImGui::UpdatePlatformWindows();
    ImGui::RenderPlatformWindowsDefault();
    glfwMakeContextCurrent(backup_current_context);
}

glfwSwapBuffers(window);

You can also store the main GL context and glfwMakeContextCurrent() on it after RenderPlatformWindowsDefault().

[JTtNinjaCode]

oh, it works!
Have you ever thought about recording some tutorial videos? Specifically about reading the imgui code and some design principles, etc. I really like this repo and hope there will be a way to learn more.

[ocornut]

There is a Getting Started guide there:
https://github.com/ocornut/imgui/wiki/Getting-Started
And many tutorial videos on the internet (although maybe IMHO are of dubious quality, people making things more complicated for themselves).

And i'm trying to better populate the Wiki over time but there's so much to do.

Glad it solved your problem!