Fix "preview file" action not working

Root cause is the PreviewSelectDialog::readyToPreviewFile signal is
emitted before it is connected to a slot.
Only single-file torrents are affected.

Closes #11315.

src/gui/previewselectdialog.cpp

@@ -101,16 +101,6 @@ PreviewSelectDialog::PreviewSelectDialog(QWidget *parent, const BitTorrent::Torr
 
     // Restore dialog state
     loadWindowState();
-
-    if (m_previewListModel->rowCount() == 1) {
-        qDebug("Torrent file only contains one file, no need to display selection dialog before preview");
-        // Only one file : no choice
-        previewButtonClicked();
-    }
-    else {
-        qDebug("Displaying media file selection dialog for preview");
-        show();
-    }
 }
 
 PreviewSelectDialog::~PreviewSelectDialog()
@@ -161,13 +151,21 @@ void PreviewSelectDialog::loadWindowState()
 
 void PreviewSelectDialog::showEvent(QShowEvent *event)
 {
-    Q_UNUSED(event);
+    // event originated from system
+    if (event->spontaneous()) {
+        QDialog::showEvent(event);
+        return;
+    }
 
     // Default size, have to be called after show(), because width is needed
     // Set Name column width to 60% of TreeView
     if (!m_headerStateInitialized) {
-        int nameSize = (m_ui->previewList->size().width() * 0.6);
+        const int nameSize = (m_ui->previewList->size().width() * 0.6);
         m_ui->previewList->header()->resizeSection(0, nameSize);
         m_headerStateInitialized = true;
     }
+
+    // Only one file, no choice
+    if (m_previewListModel->rowCount() <= 1)
+        previewButtonClicked();
 }

src/gui/transferlistwidget.cpp

@@ -506,6 +506,7 @@ void TransferListWidget::previewSelectedTorrents()
             auto *dialog = new PreviewSelectDialog(this, torrent);
             dialog->setAttribute(Qt::WA_DeleteOnClose);
             connect(dialog, &PreviewSelectDialog::readyToPreviewFile, this, &TransferListWidget::previewFile);
+            dialog->show();
         }
         else {
             QMessageBox::critical(this, tr("Unable to preview"), tr("The selected torrent does not contain previewable files"));