OpenGL is documented to flush when buffers are swapped so a glFlush() in C code (and other extra calls that flush OpenGL) can be removed. For example:

JNIEXPORT jboolean JNICALL Java_com_sun_prism_es2_EGLFBGLDrawable_nSwapBuffers
(JNIEnv *env, jclass jeglfbDrawable, jlong nativeDInfo) {
    int value;

    // Do not flush - eglSwapBuffers() is documented to already do a glFlush()
    //glFlush();
    
    DrawableInfo *dInfo = (DrawableInfo *) jlong_to_ptr(nativeDInfo);
    if (dInfo == NULL) {
        return JNI_FALSE;
    }
    PRISM_LOG("eglSwapBuffers(display=%p, surface=%p)",
              dInfo->egldisplay, dInfo->eglsurface);
    if (!eglSwapBuffers(dInfo->egldisplay, dInfo->eglsurface)) {
        fprintf(stderr, "eglSwapBuffers failed; eglGetError %d\n", eglGetError());
    }
    return JNI_TRUE;
}