chatgui: don't save scroll position when at bottom — restore to bottom on reopen

When scrollbar is at max, save lastPosMsgId=0 so restoreScrollPosition
falls through to scrollToBottom() instead of restoring a stale mid-scroll pos.

tools/chatgui/src/messagelist.cpp

@@ -288,9 +288,11 @@ void MessageList::saveChannelState() {
 
     int vpHeight = m_view->viewport()->height();
     QModelIndex topIdx = m_view->indexAt(QPoint(0, 0));
+    QScrollBar* sb = m_view->verticalScrollBar();
+    bool atBottom = sb && (sb->value() >= sb->maximum());
 
     qint64 lastPosMsgId  = 0;
-    if (topIdx.isValid())
+    if (!atBottom && topIdx.isValid())
         lastPosMsgId = topIdx.data(MsgIdRole).toLongLong();
 
     qint64 lastReadMsgId = m_lastReadMsgId;
@@ -353,7 +355,9 @@ void MessageList::updateReadPosition() {
     }
 
     QModelIndex topIdx = m_view->indexAt(QPoint(0, 0));
-    qint64 lastPosMsgId = topIdx.isValid() ? topIdx.data(MsgIdRole).toLongLong() : 0;
+    QScrollBar* sb = m_view->verticalScrollBar();
+    bool atBottom = sb && (sb->value() >= sb->maximum());
+    qint64 lastPosMsgId = (!atBottom && topIdx.isValid()) ? topIdx.data(MsgIdRole).toLongLong() : 0;
 
     DEBUG_DEBUG(DEBUG_CATEGORY_DEBUG,
         "MessageList::updateReadPosition ch=%s oldRead=%lld newRead=%lld pos=%lld lastVisRow=%d/%d",