Fix disappearing fill extrusions in zoom happening with conflation (i…

…nternal-1872)

There was incorrect reconcile replacements when having two different features, as if the count of features on borders was 1, the code was not checking both parts laying in the borders were the same feature

src/render/draw_fill_extrusion.ts

@@ -730,7 +730,7 @@ function updateBorders(context: Context, source: SourceCache, coord: OverscaledT
                 const saveIb = ib;
                 let count = 0;
                 while (true) {
-                    // Collect all parts overlapping parta on the edge, to make sure it is only one.
+                    // Collect all parts overlapping parts on the edge, to make sure it is only one.
                     assert(partB.borders);
                     const partBBorderRange = (partB.borders)[j];
                     if (partBBorderRange[0] > partABorderRange[1] - error) {
@@ -743,14 +743,18 @@ function updateBorders(context: Context, source: SourceCache, coord: OverscaledT
                     partB = nBucket.featuresOnBorder[b[ib]];
                 }
                 partB = nBucket.featuresOnBorder[b[saveIb]];
-                if (count > 1) {
+                let doReconcile = false;
+                if (count >= 1) {
                     // if it can be concluded that it is the piece of the same feature,
                     // use it, even following features (inner details) overlap on border edge.
                     assert(partB.borders);
                     const partBBorderRange = (partB.borders)[j];
                     if (Math.abs(partABorderRange[0] - partBBorderRange[0]) < error &&
                         Math.abs(partABorderRange[1] - partBBorderRange[1]) < error) {
                         count = 1;
+                        // In some cases count could be 1 but a different feature, here we make sure
+                        // we are reconciling the same feature
+                        doReconcile = true;
                         ib = saveIb + 1;
                     }
                 } else if (count === 0) {
@@ -760,7 +764,7 @@ function updateBorders(context: Context, source: SourceCache, coord: OverscaledT
                 }
 
                 const centroidB = nBucket.centroidData[partB.centroidDataIndex];
-                if (reconcileReplacementState && count === 1) {
+                if (reconcileReplacementState && doReconcile) {
                     reconcileReplacement(centroidA, centroidB);
                 }