reverse-scroll chat: start at bottom, lazy-load on scroll up

- Model shows last 40 blocks initially, positioned at end
- Scrolling near top triggers loadEarlier() which prepends 30 blocks
- Scroll position maintained after prepend via contentY adjustment
- All blocks still parsed (for searching later) but only tail rendered
- Reduced initial render cost significantly

gui/chatblockmodel.cpp

@@ -81,25 +81,33 @@ void ChatBlockModel::setRawText(const QString &text)
     emit rawTextChanged();
 }
 
-static constexpr int MAX_VISIBLE_BLOCKS = 80;
+static constexpr int INITIAL_BLOCKS = 40;
+static constexpr int LOAD_MORE_COUNT = 30;
 
 void ChatBlockModel::reparse()
 {
-    QList<ChatBlock> allBlocks = parseBlocks(m_rawText);
+    m_allBlocks = parseBlocks(m_rawText);
 
-    // Only keep the last N blocks to avoid rendering hundreds of delegates
-    QList<ChatBlock> newBlocks;
-    if (allBlocks.size() > MAX_VISIBLE_BLOCKS) {
-        newBlocks = allBlocks.mid(allBlocks.size() - MAX_VISIBLE_BLOCKS);
+    // Determine visible window: last INITIAL_BLOCKS (or expand if already showing more)
+    int desiredStart;
+    if (m_blocks.isEmpty()) {
+        // Initial load: show last N blocks
+        desiredStart = qMax(0, m_allBlocks.size() - INITIAL_BLOCKS);
     } else {
-        newBlocks = allBlocks;
+        // Streaming update: keep the same start offset, just extend the tail
+        desiredStart = m_visibleStart;
+        // But if allBlocks shrank (session switch), reset
+        if (desiredStart >= m_allBlocks.size())
+            desiredStart = qMax(0, m_allBlocks.size() - INITIAL_BLOCKS);
     }
+    m_visibleStart = desiredStart;
+
+    QList<ChatBlock> newBlocks = m_allBlocks.mid(m_visibleStart);
 
     // Incremental update: only change what differs
     int commonCount = qMin(m_blocks.size(), newBlocks.size());
-    int firstDiff = commonCount; // index of first different block
+    int firstDiff = commonCount;
 
-    // Find first block that changed (usually the last one during streaming)
     for (int i = 0; i < commonCount; ++i) {
         if (m_blocks[i].content != newBlocks[i].content ||
             m_blocks[i].type != newBlocks[i].type ||
@@ -122,7 +130,7 @@ void ChatBlockModel::reparse()
         emit dataChanged(index(i), index(i));
     }
 
-    // Append new blocks
+    // Append new blocks at end
     if (newBlocks.size() > m_blocks.size()) {
         beginInsertRows(QModelIndex(), m_blocks.size(), newBlocks.size() - 1);
         for (int i = m_blocks.size(); i < newBlocks.size(); ++i) {
@@ -132,6 +140,26 @@ void ChatBlockModel::reparse()
     }
 }
 
+bool ChatBlockModel::loadEarlier()
+{
+    if (m_visibleStart <= 0)
+        return false; // already showing everything
+
+    int loadCount = qMin(LOAD_MORE_COUNT, m_visibleStart);
+    int newStart = m_visibleStart - loadCount;
+
+    QList<ChatBlock> prepend = m_allBlocks.mid(newStart, loadCount);
+
+    beginInsertRows(QModelIndex(), 0, loadCount - 1);
+    for (int i = loadCount - 1; i >= 0; --i) {
+        m_blocks.prepend(prepend[i]);
+    }
+    m_visibleStart = newStart;
+    endInsertRows();
+
+    return true;
+}
+
 QList<ChatBlock> ChatBlockModel::parseBlocks(const QString &text) const
 {
     QList<ChatBlock> blocks;

gui/chatblockmodel.h

@@ -61,6 +61,7 @@ public:
 
     Q_INVOKABLE QString highlightedHtml(const QString &code, const QString &language) const;
     Q_INVOKABLE void copyToClipboard(const QString &text) const;
+    Q_INVOKABLE bool loadEarlier(); // returns true if more blocks were loaded
 
 signals:
     void rawTextChanged();
@@ -71,8 +72,10 @@ private:
     QVariantList parseSubBlocks(const QString &content) const;
 
     QString m_rawText;
-    QList<ChatBlock> m_blocks;
-    mutable QHash<QString, QString> m_highlightCache; // key: code+lang, value: html
+    QList<ChatBlock> m_allBlocks;   // all parsed blocks
+    QList<ChatBlock> m_blocks;      // visible window (tail of m_allBlocks)
+    int m_visibleStart = 0;         // index into m_allBlocks where visible window starts
+    mutable QHash<QString, QString> m_highlightCache;
 };
 
 #endif // CHAT_BLOCK_MODEL_H

gui/main.qml

@@ -371,13 +371,29 @@ ApplicationWindow {
                         spacing: 8
                         cacheBuffer: 200
                         model: chatModel
+                        verticalLayoutDirection: ListView.TopToBottom
 
                         property bool _scrolling: false
 
+                        Component.onCompleted: {
+                            Qt.callLater(function() { chatList.positionViewAtEnd() })
+                        }
+
                         onContentYChanged: {
                             if (_scrolling) return
                             let bottom = contentHeight - height
                             parent.atBottom = (bottom <= 0 || contentY >= bottom - 4)
+
+                            // Load earlier messages when scrolled near top
+                            if (contentY < 100 && contentHeight > height) {
+                                let oldHeight = contentHeight
+                                if (chatModel.loadEarlier()) {
+                                    // Maintain scroll position after prepend
+                                    Qt.callLater(function() {
+                                        chatList.contentY += (chatList.contentHeight - oldHeight)
+                                    })
+                                }
+                            }
                         }
                         onContentHeightChanged: {
                             if (parent.atBottom && !_scrolling) {