Prevent low vsync fps from slowing tick under old 20ms

src/overlaycontroller.cpp

@@ -540,7 +540,18 @@ void OverlayController::OnTimeoutPumpEvents()
 
         // wait for the next frame after executing our main event loop once.
         m_lastFrame = m_currentFrame;
+        m_vsyncTooLateCounter = 0;
     }
+    else if ( m_vsyncTooLateCounter >= k_nonVsyncTickRate )
+    {
+        mainEventLoop();
+        // m_lastFrame = m_currentFrame + 1 skips the next vsync frame in case
+        // it was just about to trigger, to prevent double updates faster than
+        // 11ms.
+        m_lastFrame = m_currentFrame + 1;
+        m_vsyncTooLateCounter = 0;
+    }
+    m_vsyncTooLateCounter++;
 }
 
 void OverlayController::mainEventLoop()

src/overlaycontroller.h

@@ -58,6 +58,9 @@ constexpr int k_reviveSettingsUpdateCounter = 139;
 constexpr int k_settingsTabSettingsUpdateCounter = 157;
 constexpr int k_steamVrSettingsUpdateCounter = 97;
 constexpr int k_utilitiesSettingsUpdateCounter = 19;
+// k_nonVsyncTickRate determines number of ms we wait to force the next event
+// loop tick when vsync is too late due to dropped frames.
+constexpr int k_nonVsyncTickRate = 20;
 
 class OverlayController : public QObject
 {
@@ -102,6 +105,7 @@ private:
 
     uint64_t m_currentFrame = 0;
     uint64_t m_lastFrame = 0;
+    int m_vsyncTooLateCounter = 0;
 
     // OpenVR_Init must be declared before any other class that uses OpenVR
     // function calls since objects are initialized in order of declaration in