fix: broken flamegraph

Apparently when running X11 the flamgraph doesn't clear the background
correctly. This patch resets the background brush which clears the
background correctly.

fixes: #624

src/flamegraph.cpp

@@ -673,6 +673,8 @@ FlameGraph::FlameGraph(QWidget* parent, Qt::WindowFlags flags)
 
     connect(Settings::instance(), &Settings::collapseDepthChanged, this, updateHelper);
 
+    m_scene->setBackgroundBrush(QBrush());
+
     m_scene->setItemIndexMethod(QGraphicsScene::NoIndex);
     m_view->setScene(m_scene);
     m_view->viewport()->installEventFilter(this);
@@ -1189,12 +1191,6 @@ void FlameGraph::selectItem(FrameGraphicsItem* item)
     // then layout all items below the selected on
     layoutItems(item);
 
-#if QT_VERSION <= QT_VERSION_CHECK(5, 15, 12) && QT_VERSION > QT_VERSION_CHECK(5, 15, 10)
-    // the scene background doesn't get cleared correctly when using qt 5.15.12
-    // this doesn't happen in 5.15.10 and 5.15.13
-    m_scene->setBackgroundBrush(QBrush());
-#endif
-
     // Triggers a refresh of the scene's bounding rect without going via the
     // event loop. This makes the centerOn call below work as expected in all cases.
     m_scene->sceneRect();