Merge #18121: gui: Throttle GUI update pace when -reindex

c9fe612 gui: Throttle GUI update pace when -reindex (Hennadii Stepanov)

Pull request description:

  This is grabbed from #17565.

  All **laanwj**'s and **ryanofsky**'s suggestions are implemented.

  With this PR,  the GUI does not freeze when a user runs:
  ```
  $ ./src/qt/bitcoin-qt -reindex
  ```

ACKs for top commit:
  jonasschnelli:
    utACK c9fe612

Tree-SHA512: c7be316cb73d3d286bdf8429a960f71777d13a73d059869a64e23ad276499252b561a3a5b9613c4c1ad58cc0de26283c1ec72be745c401f604eaa05f70bf7d64

Author: jonasschnelli

src/qt/clientmodel.cpp

@@ -242,8 +242,9 @@ static void BlockTipChanged(ClientModel *clientmodel, bool initialSync, int heig
         clientmodel->cachedBestHeaderHeight = height;
         clientmodel->cachedBestHeaderTime = blockTime;
     }
-    // if we are in-sync or if we notify a header update, update the UI regardless of last update time
-    if (fHeader || !initialSync || now - nLastUpdateNotification > MODEL_UPDATE_DELAY) {
+
+    // During initial sync, block notifications, and header notifications from reindexing are both throttled.
+    if (!initialSync || (fHeader && !clientmodel->node().getReindex()) || now - nLastUpdateNotification > MODEL_UPDATE_DELAY) {
         //pass an async signal to the UI thread
         bool invoked = QMetaObject::invokeMethod(clientmodel, "numBlocksChanged", Qt::QueuedConnection,
                                   Q_ARG(int, height),