Fix scrolling flickering on backwards pagination

include/TimelineView.h

@@ -111,12 +111,16 @@ private:
 	bool isInitialized = false;
 	bool isTimelineFinished = false;
 	bool isInitialSync = true;
+	bool isPaginationScrollPending_ = false;
 
-	const int SCROLL_BAR_GAP = 300;
+	const int SCROLL_BAR_GAP = 400;
 
 	int scroll_height_ = 0;
 	int previous_max_height_ = 0;
 
+	int oldPosition_;
+	int oldHeight_;
+
 	QList<PendingMessage> pending_msgs_;
 	QSharedPointer<MatrixClient> client_;
 };

src/TimelineView.cc

@@ -58,6 +58,15 @@ void TimelineView::sliderRangeChanged(int min, int max)
 
 	if (max - scroll_area_->verticalScrollBar()->value() < SCROLL_BAR_GAP)
 		scroll_area_->verticalScrollBar()->setValue(max);
+
+	if (isPaginationScrollPending_) {
+		isPaginationScrollPending_ = false;
+
+		int currentHeight = scroll_widget_->size().height();
+		int diff = currentHeight - oldHeight_;
+
+		scroll_area_->verticalScrollBar()->setValue(oldPosition_ + diff);
+	}
 }
 
 void TimelineView::scrollDown()
@@ -88,17 +97,14 @@ void TimelineView::sliderMoved(int position)
 			return;
 
 		// Prevent user from moving up when there is pagination in progress.
-		if (isPaginationInProgress_) {
-			scroll_area_->verticalScrollBar()->setValue(SCROLL_BAR_GAP);
+		// TODO: Keep a map of the event ids to filter out duplicates.
+		if (isPaginationInProgress_)
 			return;
-		}
 
 		isPaginationInProgress_ = true;
-		scroll_height_ = scroll_area_->verticalScrollBar()->value();
-		previous_max_height_ = scroll_area_->verticalScrollBar()->maximum();
 
 		// FIXME: Maybe move this to TimelineViewManager to remove the extra calls?
-		client_.data()->messages(room_id_, prev_batch_token_);
+		client_->messages(room_id_, prev_batch_token_);
 	}
 }
 
@@ -130,11 +136,15 @@ void TimelineView::addBackwardsEvents(const QString &room_id, const RoomMessages
 	// Reverse again to render them.
 	std::reverse(items.begin(), items.end());
 
+	oldPosition_ = scroll_area_->verticalScrollBar()->value();
+	oldHeight_ = scroll_widget_->size().height();
+
 	for (const auto &item : items)
 		addTimelineItem(item, TimelineDirection::Top);
 
 	prev_batch_token_ = msgs.end();
 	isPaginationInProgress_ = false;
+	isPaginationScrollPending_ = true;
 }
 
 TimelineItem *TimelineView::parseMessageEvent(const QJsonObject &event, TimelineDirection direction)