scene/surfaceitem: change refresh rate estimation to frame time estimation

This is both more direct and avoids divisions by durations that can potentially
be zero

BUG: 480971

src/core/renderloop.cpp

@@ -204,7 +204,7 @@ void RenderLoop::scheduleRepaint(Item *item)
     const bool vrr = d->presentationMode == PresentationMode::AdaptiveSync || d->presentationMode == PresentationMode::AdaptiveAsync;
     if (vrr && workspace()->activeWindow() && d->output) {
         Window *const activeWindow = workspace()->activeWindow();
-        if (activeWindow->isOnOutput(d->output) && activeWindow->surfaceItem() && item != activeWindow->surfaceItem() && activeWindow->surfaceItem()->refreshRateEstimation() >= 30) {
+        if (activeWindow->isOnOutput(d->output) && activeWindow->surfaceItem() && item != activeWindow->surfaceItem() && activeWindow->surfaceItem()->frameTimeEstimation() <= std::chrono::nanoseconds(1'000'000'000) / 30) {
             return;
         }
     }

src/scene/surfaceitem.cpp

@@ -104,13 +104,12 @@ static QRegion expandRegion(const QRegion &region, const QMargins &padding)
 void SurfaceItem::addDamage(const QRegion &region)
 {
     if (m_lastDamage) {
-        const auto diff = std::max(std::chrono::steady_clock::now() - *m_lastDamage, 10'000ns);
+        const auto diff = std::chrono::steady_clock::now() - *m_lastDamage;
         m_lastDamageTimeDiffs.push_back(diff);
         if (m_lastDamageTimeDiffs.size() > 100) {
             m_lastDamageTimeDiffs.pop_front();
         }
-        const auto average = std::accumulate(m_lastDamageTimeDiffs.begin(), m_lastDamageTimeDiffs.end(), 0ns) / m_lastDamageTimeDiffs.size();
-        m_refreshRate = 1'000'000'000ns / average;
+        m_frameTimeEstimation = std::accumulate(m_lastDamageTimeDiffs.begin(), m_lastDamageTimeDiffs.end(), 0ns) / m_lastDamageTimeDiffs.size();
     }
     m_lastDamage = std::chrono::steady_clock::now();
     m_damage += region;
@@ -261,14 +260,13 @@ void SurfaceItem::freeze()
 {
 }
 
-double SurfaceItem::refreshRateEstimation() const
+std::chrono::nanoseconds SurfaceItem::frameTimeEstimation() const
 {
     if (m_lastDamage) {
         const auto diff = std::chrono::steady_clock::now() - *m_lastDamage;
-        const double refreshRate = std::chrono::nanoseconds(1'000'000'000) / diff;
-        return std::min(m_refreshRate, refreshRate);
+        return std::max(m_frameTimeEstimation, diff);
     } else {
-        return m_refreshRate;
+        return m_frameTimeEstimation;
     }
 }
 

src/scene/surfaceitem.h

@@ -58,7 +58,7 @@ public:
 
     virtual void freeze();
 
-    double refreshRateEstimation() const;
+    std::chrono::nanoseconds frameTimeEstimation() const;
 
 Q_SIGNALS:
     void damaged();
@@ -81,7 +81,7 @@ protected:
     int m_referencePixmapCounter = 0;
     std::deque<std::chrono::nanoseconds> m_lastDamageTimeDiffs;
     std::optional<std::chrono::steady_clock::time_point> m_lastDamage;
-    double m_refreshRate = 0;
+    std::chrono::nanoseconds m_frameTimeEstimation = std::chrono::days(1000);
 };
 
 class KWIN_EXPORT SurfaceTexture