qt: Improve GUI responsiveness

QProgressDialog estimates the time the operation will take (based on
time for steps), and only shows itself if that estimate is beyond
minimumDuration. The default minimumDuration value is 4 seconds, and it
could make users think that the GUI is frozen.

Author: hebasto

src/qt/bitcoingui.cpp

@@ -1375,7 +1375,6 @@ void BitcoinGUI::showProgress(const QString &title, int nProgress)
         progressDialog = new QProgressDialog(title, QString(), 0, 100);
         GUIUtil::PolishProgressDialog(progressDialog);
         progressDialog->setWindowModality(Qt::ApplicationModal);
-        progressDialog->setMinimumDuration(0);
         progressDialog->setAutoClose(false);
         progressDialog->setValue(0);
     } else if (nProgress == 100) {

src/qt/guiutil.cpp

@@ -818,9 +818,12 @@ void PolishProgressDialog(QProgressDialog* dialog)
     // Workaround for macOS-only Qt bug; see: QTBUG-65750, QTBUG-70357.
     const int margin = TextWidth(dialog->fontMetrics(), ("X"));
     dialog->resize(dialog->width() + 2 * margin, dialog->height());
-#else
-    Q_UNUSED(dialog);
 #endif
+    // QProgressDialog estimates the time the operation will take (based on time
+    // for steps), and only shows itself if that estimate is beyond minimumDuration.
+    // The default minimumDuration value is 4 seconds, and it could make users
+    // think that the GUI is frozen.
+    dialog->setMinimumDuration(0);
 }
 
 int TextWidth(const QFontMetrics& fm, const QString& text)

src/qt/walletcontroller.cpp

@@ -207,6 +207,9 @@ void WalletControllerActivity::showProgressDialog(const QString& label_text)
     m_progress_dialog->setCancelButton(nullptr);
     m_progress_dialog->setWindowModality(Qt::ApplicationModal);
     GUIUtil::PolishProgressDialog(m_progress_dialog);
+    // The setValue call forces QProgressDialog to start the internal duration estimation.
+    // See details in https://bugreports.qt.io/browse/QTBUG-47042.
+    m_progress_dialog->setValue(0);
 }
 
 void WalletControllerActivity::destroyProgressDialog()

src/qt/walletview.cpp

@@ -330,7 +330,6 @@ void WalletView::showProgress(const QString &title, int nProgress)
         progressDialog = new QProgressDialog(title, tr("Cancel"), 0, 100);
         GUIUtil::PolishProgressDialog(progressDialog);
         progressDialog->setWindowModality(Qt::ApplicationModal);
-        progressDialog->setMinimumDuration(0);
         progressDialog->setAutoClose(false);
         progressDialog->setValue(0);
     } else if (nProgress == 100) {