StatusVector: Handle (hypothetical) situation when one image is overwritten by itself

common/StatusVector.cpp

@@ -25,11 +25,23 @@ void StatusVector::AddElement(uint32_t id, float val) {
     if (id >= content.size()) {
         content.resize(id + 1, NAN);
     }
+
+    const float old_val = content[id];
+    const bool had_old = std::isfinite(old_val);
+
     content[id] = val;
 
-    sum += val;
-    count += 1;
-    mean = sum / count;
+    if (had_old) {
+        sum += static_cast<double>(val) - static_cast<double>(old_val);
+    } else {
+        sum += val;
+        count += 1;
+    }
+
+    if (count > 0)
+        mean = static_cast<float>(sum / static_cast<double>(count));
+    else
+        mean = NAN;
 }
 
 std::optional<float> StatusVector::GetElement(uint32_t id) const {