fix deadlock dragging multiple files to timeline

A mutex is not needed because this is UI code that must be called on
the same thread. Secondly, the LongUiTask cancel invoked itself instead
of QProgressDialog::cancel(). And third, the long task progress dialog
was always canceled by the AvFormatProducerWidget and not only when a
dialog would be displayed.

src/dialogs/longuitask.cpp

@@ -17,7 +17,6 @@
 
 #include "longuitask.h"
 #include "mainwindow.h"
-#include <QMutexLocker>
 
 static const char* kLongUiTask = "LongUiTask";
 static QMutex g_mutex;
@@ -31,13 +30,11 @@ LongUiTask::LongUiTask(QString title)
     setWindowModality(Qt::ApplicationModal);
     setMinimumDuration(2000);
     setRange(0, 0);
-    QMutexLocker locker(&g_mutex);
     g_instance = this;
 }
 
 LongUiTask::~LongUiTask()
 {
-    QMutexLocker locker(&g_mutex);
     g_instance = nullptr;
 }
 
@@ -51,8 +48,7 @@ void LongUiTask::reportProgress(QString text, int value, int max)
 
 void LongUiTask::cancel()
 {
-    QMutexLocker locker(&g_mutex);
     if (g_instance) {
-        g_instance->cancel();
+        g_instance->QProgressDialog::cancel();
     }
 }

src/widgets/avformatproducerwidget.cpp

@@ -518,9 +518,9 @@ void AvformatProducerWidget::onFrameDecoded()
     ui->syncSlider->setValue(qRound(m_producer->get_double("video_delay") * 1000.0));
 
     if (Settings.showConvertClipDialog() && !m_producer->get_int(kShotcutSkipConvertProperty)) {
-        LongUiTask::cancel();
-        m_producer->set(kShotcutSkipConvertProperty, true);
         if (isVariableFrameRate) {
+            m_producer->set(kShotcutSkipConvertProperty, true);
+            LongUiTask::cancel();
             MLT.pause();
             LOG_INFO() << resource << "is variable frame rate";
             TranscodeDialog dialog(tr("This file is variable frame rate, which is not reliable for editing. "
@@ -534,6 +534,8 @@ void AvformatProducerWidget::onFrameDecoded()
             convert(dialog);
         }
         if (QFile::exists(resource) && !MLT.isSeekable(m_producer.data())) {
+            m_producer->set(kShotcutSkipConvertProperty, true);
+            LongUiTask::cancel();
             MLT.pause();
             LOG_INFO() << resource << "is not seekable";
             TranscodeDialog dialog(tr("This file does not support seeking and cannot be used for editing. "