SDL2使用单独的绘制线程后绘制的东西大小会虽窗口大小自动改变

代码：
#include<stdbool.h>
#include<SDL2/SDL.h>
const int FPS=60;
const char*title="Test";
int width=640,height=480;
bool isRun;
SDL_Window*win=NULL;
SDL_Renderer*ren=NULL;
bool initSDL(){
if(SDL_Init(SDL_INIT_VIDEO)!=0){
SDL_LogError(SDL_LOG_CATEGORY_APPLICATION,"SDL2 init error:%s",SDL_GetError());
return false;
}
return true;
}
void quit(){
if(ren!=NULL){
SDL_DestroyRenderer(ren);
}
if(win!=NULL){
SDL_DestroyWindow(win);
}
SDL_Quit();
}
bool createSDL(){
win=SDL_CreateWindow(title,SDL_WINDOWPOS_UNDEFINED,SDL_WINDOWPOS_UNDEFINED,width,height,SDL_WINDOW_SHOWN|SDL_WINDOW_RESIZABLE);
if(win==NULL){
SDL_LogError(SDL_LOG_CATEGORY_APPLICATION,"SDL2 create window error:%s",SDL_GetError());
return false;
}
ren=SDL_CreateRenderer(win,-1,SDL_RENDERER_ACCELERATED|SDL_RENDERER_PRESENTVSYNC);
if(ren==NULL){
SDL_LogError(SDL_LOG_CATEGORY_APPLICATION,"SDL2 create window error:%s",SDL_GetError());
return false;
}
return true;
}
void render(){
/*背景颜色*/
SDL_SetRenderDrawColor(ren,18,18,18,0xFF);
SDL_RenderClear(ren);
/*画正方形*/
SDL_SetRenderDrawColor(ren,0xFF,0,0,0xFF);
SDL_Rect rt={(width-100)/2,(height-100)/2,100,100};/*奇怪，我宽和高都是100，怎么在改变窗口大小时宽高也会改变？*/
SDL_RenderFillRect(ren,&rt);
SDL_RenderPresent(ren);
}
/*绘制线程*/
int drawing(void*data){
unsigned long long start;
int waitTime;
while(isRun){
/*控制帧率*/
start=SDL_GetPerformanceCounter();
render();
waitTime=1000/FPS-(SDL_GetPerformanceCounter()-start)*1000/SDL_GetPerformanceFrequency();
if(waitTime<0 || waitTime>=500){
waitTime=1000/FPS;
}
SDL_Delay(waitTime);
}
return 0;
}
void handleEvent(){
SDL_Event event;
SDL_WaitEvent(&event);
if(true){
switch(event.type){
case SDL_QUIT:
isRun=false;
break;
case SDL_WINDOWEVENT:
switch(event.window.event){
case SDL_WINDOWEVENT_SIZE_CHANGED:
SDL_GetWindowSize(win,&width,&height);
SDL_Log("size change to %d,%d\n",width,height);
break;
case SDL_WINDOWEVENT_RESIZED:
width=event.window.data1;
height=event.window.data2;
SDL_Log("resize to %d,%d\n",width,height);
break;
default:
break;
}
default:
break;
}
}
}
int main(int argc,char**argv){
if(!initSDL()){
return 1;
}
isRun=createSDL();
SDL_Thread*th=SDL_CreateThread(drawing,"drawing",NULL);
while(isRun){
handleEvent();
}
SDL_WaitThread(th,NULL);
quit();
return 0;
}
结果：

为什么会这样？求大佬解释