Ensure stable sort order when sorting by size

Folders with equal size caused the sort operation to become unstable,
as the result of lessThan was non-deterministic.

We need the fallback mechanisms at the bottom of the function to
resovle the situation and provide a stable sort order.

This also fixes expanding the contents of a folder into the wrong
parent.

BUG: 433247
FIXED-IN: 21.08

src/kitemviews/kfileitemmodel.cpp

@@ -1798,17 +1798,25 @@ int KFileItemModel::sortRoleCompare(const ItemData* a, const ItemData* b, const
             auto valueB = b->values.value("count");
             if (valueA.isNull()) {
                 if (valueB.isNull()) {
-                    return 0;
+                    result = 0;
+                    break;
                 } else {
-                    return -1;
+                    result = -1;
+                    break;
                 }
             } else if (valueB.isNull()) {
-                return +1;
+                result = +1;
+                break;
             } else {
                 if (valueA.toLongLong() < valueB.toLongLong()) {
-                    return -1;
+                    result = -1;
+                    break;
+                } else if (valueA.toLongLong() > valueB.toLongLong()) {
+                    result = +1;
+                    break;
                 } else {
-                    return +1;
+                    result = 0;
+                    break;
                 }
             }
         }