Title says it all.
The code works with the XPeekEvent right before the while XCheckTypedEvent but without that the conditional evaluates to false and I have no idea why that might be.

#include <X11/Xlib.h>

#define MAX(a, b) ((a) > (b) ? (a) : (b))
#define false 0
#define true 1


int main(void)
{
    Display* dpy;
    XWindowAttributes attr;

    // state of cursor at the start of move/resize
    XButtonEvent cur_start;

    // return failure status if we can't connect
    if (!(dpy = XOpenDisplay(0x0))) return 1;

    char* keys_to_grab[] = {"F1"};
    for (size_t i = 0; i < sizeof(keys_to_grab) / sizeof(keys_to_grab[0]); i++)
    {
        XGrabKey(dpy, XKeysymToKeycode(dpy, XStringToKeysym(keys_to_grab[i])), Mod1Mask,
            DefaultRootWindow(dpy), True, GrabModeAsync, GrabModeAsync);
    }

    XGrabButton(dpy, 1, Mod1Mask, DefaultRootWindow(dpy), True,
            ButtonPressMask|ButtonReleaseMask|PointerMotionMask, GrabModeAsync, GrabModeAsync, None, None);

    XGrabButton(dpy, 3, Mod1Mask, DefaultRootWindow(dpy), True,
            ButtonPressMask|ButtonReleaseMask|PointerMotionMask, GrabModeAsync, GrabModeAsync, None, None);

    cur_start.subwindow = None;

    for(;;)
    {
        XNextEvent(dpy, &ev);
        switch(ev.type)
        {
            case KeyPress:
                if (ev.xkey.subwindow != None)
                    XRaiseWindow(dpy, ev.xkey.subwindow);
                break;

            case ButtonPress:
                if (ev.xbutton.subwindow != None)
                    cur_start = ev.xbutton;
                break;
                
            case MotionNotify:
                if (cur_start.subwindow != None)
                {
                    XPeekEvent(dpy, &ev);
                    while(XCheckTypedEvent(dpy, MotionNotify, &ev))
                    {
                        int xdiff = ev.xbutton.x_root - cur_start.x_root;
                        int ydiff = ev.xbutton.y_root - cur_start.y_root;
                       XMoveWindow(dpy, cur_start.subwindow,
                                attr.x + (cur_start.button==1 ? xdiff : 0),
                                attr.y + (cur_start.button==1 ? ydiff : 0));
                    }
                }
                break;

            case ButtonRelease:
                cur_start.subwindow = None;
                break;

            default:
                break;
        }
    }
}

This is how I run the code: xinit ./xinitrc -- /usr/bin/Xephyr :100 -ac -screen 500x500 -host-cursor
With this as the xinitrc:

xclock &
xeyes &
sleep 2 && xterm &

exec ./a.out

and by “works”, I mean I can drag the windows around.