Use native scrollbar in the timeline

resources/styles/nheko-dark.qss

@@ -17,6 +17,11 @@ QLabel {
 TimelineView,
 TimelineView > * {
     background-color: #202228;
+    border: none;
+}
+
+#scroll_widget {
+    background-color: #202228;
 }
 
 QuickSwitcher {

resources/styles/nheko.qss

@@ -17,6 +17,11 @@ QLabel {
 TimelineView,
 TimelineView > * {
     background-color: white;
+    border: none;
+}
+
+#scroll_widget {
+    background-color: white;
 }
 
 QuickSwitcher {

resources/styles/system.qss

@@ -19,6 +19,7 @@ TimelineView > *,
 TimelineItem,
 TimelineItem > * {
     background-color: palette(window);
+    border: none;
 }
 
 QuickSwitcher {

src/timeline/TimelineView.cpp

@@ -506,10 +506,8 @@ TimelineView::init()
         scroll_area_->setWidgetResizable(true);
         scroll_area_->setHorizontalScrollBarPolicy(Qt::ScrollBarAlwaysOff);
 
-        scrollbar_ = new ScrollBar(scroll_area_);
-        scroll_area_->setVerticalScrollBar(scrollbar_);
-
         scroll_widget_ = new QWidget(this);
+        scroll_widget_->setObjectName("scroll_widget");
 
         scroll_layout_ = new QVBoxLayout(scroll_widget_);
         scroll_layout_->setContentsMargins(4, 0, 15, 15);

src/timeline/TimelineViewManager.cpp

@@ -32,9 +32,7 @@
 
 TimelineViewManager::TimelineViewManager(QWidget *parent)
   : QStackedWidget(parent)
-{
-        setStyleSheet("border: none;");
-}
+{}
 
 void
 TimelineViewManager::updateReadReceipts(const QString &room_id,