SDL3: OpenGL Borderless Window Becomes Exclusive Fullscreen on SDL_GL_SwapWindow (Windows 11)

[vittorioromeo]

Creating a borderless OpenGL window (SDL_WINDOW_OPENGL | SDL_WINDOW_BORDERLESS) at the native display resolution on Windows 11 results in the window unexpectedly switching to an exclusive fullscreen mode upon the first call to SDL_GL_SwapWindow. This behavior is not desired; the expectation is a standard borderless fullscreen windowed mode.

• OS: Windows 11 24H2
• GPU: NVIDIA RTX 4090
• Graphics Driver Version: 32.0.15.7247

---

MCVE:

#include <SDL3/SDL.h>

int main()
{
    SDL_InitSubSystem(SDL_INIT_VIDEO);

    SDL_WindowFlags flags = {};
    flags |= SDL_WINDOW_OPENGL;
    flags |= SDL_WINDOW_RESIZABLE;
    flags |= SDL_WINDOW_BORDERLESS;

    SDL_Window *w = SDL_CreateWindow("", 3440, 1440, flags); // use fullscreen resolution here
    SDL_GLContext wCtx = SDL_GL_CreateContext(w);

    while (true) {
        SDL_Event event;
        while (SDL_PollEvent(&event)) { }

        SDL_GL_SwapWindow(w); // commenting this line solves the issue
    }
}

---

Expected Behavior:

The application should display a borderless window that perfectly fills the native screen resolution (e.g., 3440x1440). This window should behave like standard borderless fullscreen windowed mode:

• It covers the entire screen, including the taskbar area.
• Alt-tabbing works smoothly without display mode changes.
• System overlays (like volume controls) appear correctly.
• No screen flickering associated with a display mode switch occurs.

---

Actual Behavior:

Upon the first call to SDL_GL_SwapWindow, the window enters an exclusive fullscreen-like mode:

• There is a brief screen flicker or blanking, suggesting a display mode change or swapchain takeover.
• Alt-tabbing becomes sluggish.
• Overlays do not render over the application.

---

Additional Context & Workarounds Tried:

• The trigger seems to be specifically the SDL_GL_SwapWindow call. Before this call, the window exists but doesn't exhibit exclusive behavior.
• Creating the window with slightly smaller dimensions (e.g., mode.w, mode.h - 1) avoids the exclusive fullscreen switch, but results in the Windows taskbar being rendered on top of the application window, which is also undesirable.
• Creating a standard windowed window first and then calling SDL_SetWindowFullscreen(window, true) also results in the same undesirable exclusive fullscreen behavior.
• Disabling "Fullscreen Optimizations" in the Windows compatibility settings for the executable does not prevent this behavior.
• Setting different VSync modes via SDL_GL_SetSwapInterval(0/1/-1) does not prevent this behavior.

[slouken]

This is intentional OpenGL driver behavior to increase performance and not something SDL can affect.

[vittorioromeo]

This is intentional OpenGL driver behavior to increase performance and not something SDL can affect.

@slouken: I honestly think that closing this issue without any chance of discussion is very premature and not in the best interests of your users. Creating a fullscreen borderless window is an extremely common requirement for modern games and applications.

Furthermore, this is definitely an SDL quirk, as I was able to achieve my goals easily with SFML, on the exact same machine and environment (in fact, I just reproduced it).

[vittorioromeo]

@slouken: This is the relevant code in my SFML fork that seems to make borderless fullscreen work flawlessly:

SetWindowLongPtr(m_handle, GWL_STYLE, WS_OVERLAPPED | WS_CLIPCHILDREN | WS_CLIPSIBLINGS);
SetWindowLongPtr(m_handle, GWL_EXSTYLE, WS_EX_APPWINDOW);

SetWindowPos(m_handle, HWND_TOP, left, top, width, height, SWP_FRAMECHANGED | SWP_NOOWNERZORDER | SWP_SHOWWINDOW);

Could this issue please be reopened?

[vittorioromeo]

Here's a MCVE that fixes the issue on SDL3 as well:

#include <SDL3/SDL.h>
#include <windows.h>

int main()
{
    SDL_InitSubSystem(SDL_INIT_VIDEO);

    SDL_WindowFlags flags = {};
    flags |= SDL_WINDOW_OPENGL;
    flags |= SDL_WINDOW_RESIZABLE;
    flags |= SDL_WINDOW_BORDERLESS;

    SDL_Window *w = SDL_CreateWindow("title", 3440, 1440, flags);

    SDL_PropertiesID props = SDL_GetWindowProperties(w);

    void *hwnd = SDL_GetPointerProperty(props, SDL_PROP_WINDOW_WIN32_HWND_POINTER, NULL);
    SetWindowLongPtr(hwnd, GWL_STYLE, WS_OVERLAPPED | WS_CLIPCHILDREN | WS_CLIPSIBLINGS);
    SetWindowLongPtr(hwnd, GWL_EXSTYLE, WS_EX_APPWINDOW);
    SetWindowPos(hwnd, HWND_TOP, 0, 0, 3440, 1440, SWP_FRAMECHANGED | SWP_NOOWNERZORDER | SWP_SHOWWINDOW);

    SDL_GLContext wCtx = SDL_GL_CreateContext(w);

    while (true) {
        SDL_Event event;
        while (SDL_PollEvent(&event)) {
        }

        SDL_GL_SwapWindow(w);
    }
}