findTopCellItem() handle underflow (f20-3)

Meter.c

@@ -386,7 +386,6 @@ static uint8_t GraphMeterMode_findTopCellItem(const Meter* this, double scaledTo
    assert(topCell < graphHeight);
 
    double valueSum = 0.0;
-   bool maxValueIsArea = false;
    double maxValue = 0.0;
    uint8_t topCellItem = this->curItems - 1;
    for (uint8_t i = 0; i < this->curItems && valueSum < DBL_MAX; i++) {
@@ -410,35 +409,33 @@ static uint8_t GraphMeterMode_findTopCellItem(const Meter* this, double scaledTo
       // Find the item that occupies the largest area of the top cell.
       // Favor item with higher index in case of a tie.
 
-      double area = (value / scaledTotal) * (double)(int)graphHeight;
-      assert(area >= 0.0);
-
-      if (topCellItem > i) {
-         assert(!maxValueIsArea);
-         assert(maxValue <= 0.0);
-
+      if (topCell > 0) {
          double topPoint = (valueSum / scaledTotal) * (double)(int)graphHeight;
          assert(topPoint >= 0.0);
 
-         if (topCell > 0 && !(topPoint > (double)(int)topCell))
+         if (!(topPoint > (double)(int)topCell))
             continue;
 
-         if (area > topPoint - (double)(int)topCell) {
-            maxValueIsArea = true;
-            maxValue = topPoint - (double)(int)topCell;
-         } else {
-            maxValue = value;
-         }
-         topCellItem = i;
-      } else if (maxValueIsArea) {
+         double area = (value / scaledTotal) * (double)(int)graphHeight;
+         assert(area >= 0.0);
+
+         if (area > topPoint - (double)(int)topCell)
+            area = topPoint - (double)(int)topCell;
+
+         // Note: The FP default rounding mode (i.e. to nearest) ensures any
+         // "area" that could win to have a number at least (DBL_EPSILON / 2).
+
          if (area >= maxValue) {
-            maxValueIsArea = false;
+            maxValue = area;
+            topCellItem = i;
+         }
+      } else {
+         // Compare "value" directly. It is possible for an "area" to
+         // underflow here, and still wins as the largest area.
+         if (value >= maxValue) {
             maxValue = value;
             topCellItem = i;
          }
-      } else if (value >= maxValue) {
-         maxValue = value;
-         topCellItem = i;
       }
    }
    return topCellItem;