use mutex to avoid window destroy deadlock, fix vertical slider to increase value upward

src/main.cpp

@@ -33,6 +33,7 @@ std::thread hmd_renderer;
 std::thread renderer;
 int running = -1;
 std::mutex render_mutex;
+std::mutex wnd_mutex;
 std::condition_variable render_cv;
 
 int gl_count = 0;
@@ -646,8 +647,13 @@ int main(int argc, char** argv)
             {
                 static wchar_t title_fps[512];
                 swprintf_s(title_fps, L"%s - %d fps", window_title, frames);
-                if (running)
+
+                // lock if 
+                if (wnd_mutex.try_lock())
+                {
                     SetWindowText(hWnd, title_fps);
+                    wnd_mutex.unlock();
+                }
                 one_sec = 0;
                 frames = 0;
             }
@@ -806,10 +812,18 @@ LRESULT CALLBACK WndProc(HWND hWnd, UINT msg, WPARAM wp, LPARAM lp)
         {
             running = 0;
             render_cv.notify_all();
-            if (renderer.joinable())
-                renderer.join();
-            if (hmd_renderer.joinable())
-                hmd_renderer.join();
+
+            {
+                // avoid deadlock
+                // nobody should call windows API on this window at this time
+                std::lock_guard<std::mutex> lock(wnd_mutex);
+
+                if (renderer.joinable())
+                    renderer.join();
+                if (hmd_renderer.joinable())
+                    hmd_renderer.join();
+            }
+
             App::I.terminate();
         }
         break;