Drastic decrease in performance when running full screen vs almost full screen

[bill-connelly]

GLFW 3.3.8-1
Raspberry Pi 5.
Raspberry Pi OS: Linux raspberrypi 6.6.28+rpt-rpi-v8 #1 SMP PREEMPT Debian 1:6.6.28-1+rpt1 (2024-04-22) aarch64 GNU/Linux
Windowing: Wayland

My monitors native resolution is 1920 x 1200. If I make a window that is just not full screen (1920, 1180) and then repeatedly call glfwSwapBuffers() with no kind of rendering and time how long between buffer swaps, I get about 0.2 milliseconds, i.e. 5000 FPS

However, if I make the window full screen, and perform the same test I get a handful of frames with a 0.2 inter-frame time. Then it drops to 33.3 millisecond (which is very curiously, almost exactly 2 refreshes), i.e. 30 FPS

I appreciate I am on some atypical hardware, so I'm happy to try to look into anything.

My test code is as follows:

#include <stdio.h>
#include <GLFW/glfw3.h>
#include <sys/time.h>

struct timeval tv;
unsigned long
  frameCount = 0,
  time_in_micros,
  oldTime;

int main(void) {

    glfwInit();

    GLFWmonitor* primaryMonitor = glfwGetPrimaryMonitor();
    const GLFWvidmode* mode = glfwGetVideoMode(primaryMonitor);
 
    //glfwWindowHint(GLFW_REFRESH_RATE, mode->refreshRate); //This makes no difference
    
    //Swapping between these two versions are what causes problems.
    //GLFWwindow* window = glfwCreateWindow(1920, 1180, "My Title", NULL, NULL);
    GLFWwindow* window = glfwCreateWindow(1920, 1200, "My Title", primaryMonitor, NULL);

    glfwMakeContextCurrent(window);
    glfwSwapInterval(0);

    while (!glfwWindowShouldClose(window)) {
        frameCount++;
        glfwSwapBuffers(window);
        if (frameCount % 10 == 0) {
            oldTime = time_in_micros;
            gettimeofday(&tv,NULL);
            time_in_micros = 1000000 * tv.tv_sec + tv.tv_usec;
            printf("Interframe time is %f\n", (float)(time_in_micros-oldTime)/10000);
        }
    }

    glfwTerminate();
    return 0;
}