fix performance: cache subBlocks and syntax highlighting

- Cache parseSubBlocks per block (only reparse when content changes)
- Cache highlightedHtml results (code+language key)
- Prevents expensive KSyntaxHighlighting and regex on every render
- Also fixes scroll loop with _scrolling guard

gui/chatblockmodel.cpp

@@ -54,7 +54,11 @@ QVariant ChatBlockModel::data(const QModelIndex &index, int role) const
     case ContentRole:
         return block.content;
     case SubBlocksRole:
-        return parseSubBlocks(block.content);
+        if (block.cachedContent != block.content) {
+            block.cachedSubBlocks = parseSubBlocks(block.content);
+            block.cachedContent = block.content;
+        }
+        return block.cachedSubBlocks;
     }
     return {};
 }
@@ -251,6 +255,12 @@ QVariantList ChatBlockModel::parseSubBlocks(const QString &content) const
 
 QString ChatBlockModel::highlightedHtml(const QString &code, const QString &language) const
 {
+    // Check cache first
+    QString cacheKey = language + "\x1f" + code;
+    auto cached = m_highlightCache.find(cacheKey);
+    if (cached != m_highlightCache.end())
+        return *cached;
+
     auto def = repo().definitionForName(language);
     if (!def.isValid()) {
         // Try by file extension
@@ -258,7 +268,9 @@ QString ChatBlockModel::highlightedHtml(const QString &code, const QString &lang
     }
     if (!def.isValid()) {
         // Return escaped plain text
-        return code.toHtmlEscaped().replace('\n', "<br>");
+        QString plain = code.toHtmlEscaped().replace('\n', "<br>");
+        m_highlightCache.insert(cacheKey, plain);
+        return plain;
     }
 
     // Determine theme based on current palette
@@ -298,6 +310,7 @@ QString ChatBlockModel::highlightedHtml(const QString &code, const QString &lang
             html += "<br>";
     }
 
+    m_highlightCache.insert(cacheKey, html);
     return html;
 }
 

gui/chatblockmodel.h

@@ -20,6 +20,8 @@ struct ChatBlock {
     Type type;
     QString name;    // tool/call name, empty for others
     QString content; // raw text content
+    mutable QVariantList cachedSubBlocks; // cached parseSubBlocks result
+    mutable QString cachedContent;        // content when subBlocks were cached
 };
 
 // A sub-block within a markdown block (prose, code fence, diff)
@@ -70,6 +72,7 @@ private:
 
     QString m_rawText;
     QList<ChatBlock> m_blocks;
+    mutable QHash<QString, QString> m_highlightCache; // key: code+lang, value: html
 };
 
 #endif // CHAT_BLOCK_MODEL_H