perf(AcpMessageWidget): debounce markdown re-renders during streaming

Parsing markdown on every streamed chunk is O(N) per call, which stalls
the UI thread on long or table-heavy replies where the full payload is
re-parsed each time.

An 80 ms single-shot timer coalesces rapid chunks into a single render
pass. setText bypasses the timer and renders immediately since wholesale
replacements represent settled state, not an in-progress stream.

Author: nullmastermind

src/widgets/AcpMessageWidget.cpp

@@ -29,6 +29,7 @@
 #include <QTextBrowser>
 #include <QTextCursor>
 #include <QTextDocument>
+#include <QTimer>
 #include <QToolButton>
 #include <QVBoxLayout>
 
@@ -132,17 +133,47 @@ AcpMessageWidget::AcpMessageWidget(QString role, QWidget *parent)
         configureBubbleBrowser(m_browser);
         m_layout->addWidget(m_browser);
     }
+
+    // Debounce assistant/thought re-renders so streamed chunks don't drown the
+    // UI thread in markdown parsing — especially severe for table-heavy
+    // replies where every chunk re-parses the whole payload.
+    m_rerenderTimer = new QTimer(this);
+    m_rerenderTimer->setSingleShot(true);
+    m_rerenderTimer->setInterval(80);
+    connect(m_rerenderTimer, &QTimer::timeout, this, &AcpMessageWidget::rerender);
+}
+
+void AcpMessageWidget::scheduleRerender()
+{
+    if (m_rerenderTimer) {
+        if (!m_rerenderTimer->isActive()) {
+            m_rerenderTimer->start();
+        }
+    } else {
+        rerender();
+    }
+}
+
+void AcpMessageWidget::flushRerender()
+{
+    if (m_rerenderTimer && m_rerenderTimer->isActive()) {
+        m_rerenderTimer->stop();
+    }
 }
 
 void AcpMessageWidget::appendChunk(const QString &chunk)
 {
     m_text += chunk;
-    rerender();
+    scheduleRerender();
 }
 
 void AcpMessageWidget::setText(const QString &fullText)
 {
     m_text = fullText;
+    // Wholesale replacements are usually terminal states (compaction done,
+    // model rewrote in place) — render immediately so the user sees the
+    // settled state rather than waiting for the debounce.
+    flushRerender();
     rerender();
 }
 

src/widgets/AcpMessageWidget.h

@@ -27,6 +27,7 @@
 #include "AcpProtocol.h"
 
 class QTextBrowser;
+class QTimer;
 class QToolButton;
 class QLabel;
 class QVBoxLayout;
@@ -74,6 +75,8 @@ class AcpMessageWidget : public QFrame
     void rerender();
     void refitBrowserHeight();
     void applyCollapsed(bool collapsed);
+    void scheduleRerender();
+    void flushRerender();
 
     QString m_role;
     QString m_text;
@@ -82,6 +85,10 @@ class AcpMessageWidget : public QFrame
     QTextBrowser *m_browser = nullptr;     // assistant + non-thought rendered widgets
     QToolButton *m_thoughtHeader = nullptr; // thought role
     QVBoxLayout *m_layout = nullptr;
+    // Debounce timer for assistant markdown re-renders. setMarkdown on a long
+    // table-bearing payload is O(N) per call; without debouncing, every
+    // streamed chunk re-parses the whole document and the UI thread stalls.
+    QTimer *m_rerenderTimer = nullptr;
 
     // User role: image thumbnails kept alongside their original pixmap so we
     // can rescale to bubble width on resize without quality loss.