Fix bestThread selection

If multiple threads have the same best move,
pick the thread with the largest contribution to the confidence vote.
This thread will later be used to display PV, so this patch is
about user-friendliness and/or least surprises, it non-functional for playing strenght.

closes #4246

No functional change

src/thread.cpp

@@ -221,11 +221,14 @@ Thread* ThreadPool::get_best_thread() const {
         minScore = std::min(minScore, th->rootMoves[0].score);
 
     // Vote according to score and depth, and select the best thread
+    auto thread_value = [minScore](Thread* th) {
+            return (th->rootMoves[0].score - minScore + 14) * int(th->completedDepth);
+        };
+
     for (Thread* th : *this)
-    {
-        votes[th->rootMoves[0].pv[0]] +=
-            (th->rootMoves[0].score - minScore + 14) * int(th->completedDepth);
+        votes[th->rootMoves[0].pv[0]] += thread_value(th);
 
+    for (Thread* th : *this)
         if (abs(bestThread->rootMoves[0].score) >= VALUE_TB_WIN_IN_MAX_PLY)
         {
             // Make sure we pick the shortest mate / TB conversion or stave off mate the longest
@@ -236,9 +239,8 @@ Thread* ThreadPool::get_best_thread() const {
                  || (   th->rootMoves[0].score > VALUE_TB_LOSS_IN_MAX_PLY
                      && (   votes[th->rootMoves[0].pv[0]] > votes[bestThread->rootMoves[0].pv[0]]
                          || (   votes[th->rootMoves[0].pv[0]] == votes[bestThread->rootMoves[0].pv[0]]
-                             && th->rootMoves[0].pv.size() > bestThread->rootMoves[0].pv.size()))))
+                             && thread_value(th) > thread_value(bestThread)))))
             bestThread = th;
-    }
 
     return bestThread;
 }