Why does double buffering cause more lag and flickering when resizing the window, using OpenGL and SDL?

Question

I'm using OpenGL and SDL to scale a software-rendered framebuffer of a fixed size (320x240) to the client window size while maintaining the ratio by putting black bars on each side if necessary.

When I use single buffering, the rendering is smooth when I resize the window. However enabling double buffering causes a lot of flickering, and the scaling gets a bit laggy to follow the changed window size.

In practice, I'd prefer double buffering because changing the window size doesn't happen all the time, and otherwise double buffering provides smoother animation, but I'd still like to know the cause of the lagginess and flickering when resizing the window with double buffering.

The code is stripped as much as possible to the relevant part of this question.

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This seems to be a problem of SDL, or at least of its Linux backend. The same program with SDL swapped to freeglut handles double buffering smoothly when resizing the window (pastebin).

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SDL wasn't wrong here. See the comments section.

#include <stdio.h>
#include <stdint.h>
#include <assert.h>
#include <math.h>
#include <SDL2/SDL.h>
#include <GL/gl.h>

#define WindowW 800
#define WindowH 600
#define FbW 320
#define FbH 240
#define FbTexW 0x200
#define FbTexH 0x100
static const float VertexCoord[] = {0, 0, FbTexW, 0, 0, FbTexH, FbTexW, FbTexH};

static void initGl() {
  glEnable(GL_TEXTURE_2D);
  glEnableClientState(GL_VERTEX_ARRAY);
  glClearColor(0, 0, 0, 0);
  glShadeModel(GL_FLAT);
  glOrtho(0, FbW, FbH, 0, 1, -1);
  glVertexPointer(2, GL_FLOAT, 0, VertexCoord);
}

static void resize(int w, int h) {
  int p = w * FbH;
  int q = h * FbW;
  if (p > q) {
    float w_ = (float)q / FbH;
    glViewport((int)roundf((w - w_) * 0.5f), 0, (int)roundf(w_), h);
  } else {
    float h_ = (float)p / FbW;
    glViewport(0, (int)roundf((h - h_) * 0.5f), w, (int)roundf(h_));
  }
}

static void draw() {
  glClear(GL_COLOR_BUFFER_BIT);
  glDrawArrays(GL_TRIANGLE_STRIP, 0, 4);
}

int main(int argc, char **argv) {
  (void)argc, (void)argv;
  assert(!SDL_Init(SDL_INIT_VIDEO));
  SDL_GL_SetAttribute(SDL_GL_DOUBLEBUFFER, 1); // 1 -> double, 0 -> single
  SDL_Window *win = SDL_CreateWindow("test", SDL_WINDOWPOS_UNDEFINED,
    SDL_WINDOWPOS_UNDEFINED, WindowW, WindowH,
    SDL_WINDOW_RESIZABLE | SDL_WINDOW_OPENGL);
  assert(win);
  SDL_GLContext gl = SDL_GL_CreateContext(win);
  assert(gl);
  initGl();
  for (;;) {
    SDL_Event event;
    assert(SDL_WaitEvent(&event));
    switch (event.type) {
    case SDL_QUIT:
      SDL_GL_DeleteContext(gl);
      SDL_DestroyWindow(win);
      SDL_Quit();
      return 0;
    case SDL_WINDOWEVENT:
      if (event.window.event != SDL_WINDOWEVENT_SIZE_CHANGED) {
        int w, h;
        SDL_GL_GetDrawableSize(win, &w, &h);
        resize(w, h);
      }
    }
    draw();
    SDL_GL_SwapWindow(win);
  }
}