Fix progressive input lag from per-frame heap allocations and redundant font metrics

Three targeted optimizations in the terminal rendering hot path:

1. Replace per-frame heap allocations with pre-allocated buffers
   - updateImage() allocated/freed wchar_t[] and char[] arrays every frame
   - Over long sessions this caused heap fragmentation, progressively
     slowing allocation and increasing input latency
   - Now uses std::vector member variables that resize only on terminal
     dimension changes

2. Hoist QFontMetrics construction out of per-line loop
   - QFontMetrics was recreated for every line in updateImage(), up to
     50+ times per frame for a typical terminal
   - Now created once per updateImage() call

3. Skip horizontalAdvance() for fixed-width fonts
   - For monospace terminal fonts (_fixedFont == true), character width
     is constant and already known as _fontWidth
   - Eliminates expensive font shaping engine lookups in both
     updateImage() and drawContents()

Author: yokotoka

lib/TerminalDisplay.cpp

@@ -1205,9 +1205,14 @@ void TerminalDisplay::updateImage()
   const int linesToUpdate = qMin(this->_lines, qMax(0,lines  ));
   const int columnsToUpdate = qMin(this->_columns,qMax(0,columns));
 
-  wchar_t *disstrU = new wchar_t[columnsToUpdate];
-  char *dirtyMask = new char[columnsToUpdate+2];
+  if (static_cast<int>(_updateDisstrU.size()) < columnsToUpdate)
+      _updateDisstrU.resize(columnsToUpdate);
+  if (static_cast<int>(_updateDirtyMask.size()) < columnsToUpdate + 2)
+      _updateDirtyMask.resize(columnsToUpdate + 2);
+  wchar_t *disstrU = _updateDisstrU.data();
+  char *dirtyMask = _updateDirtyMask.data();
   QRegion dirtyRegion;
+  QFontMetrics fm(font());
 
   for (y = 0; y < linesToUpdate; ++y)
   {
@@ -1229,7 +1234,6 @@ void TerminalDisplay::updateImage()
         }
     }
 
-    QFontMetrics fm(font());
     if (!_resizing) // not while _resizing, we're expecting a paintEvent
     for (x = 0; x < columnsToUpdate; ++x)
     {
@@ -1249,7 +1253,7 @@ void TerminalDisplay::updateImage()
         disstrU[p++] = c; //fontMap(c);
         bool lineDraw = isLineChar(newLine[x+0]);
         bool doubleWidth = (x+1 == columnsToUpdate) ? false : (newLine[x+1].character == 0);
-        int charWidth = fm.horizontalAdvance(QChar(c));
+        int charWidth = _fixedFont ? _fontWidth : fm.horizontalAdvance(QChar(c));
         bool bigWidth = _fixedFont && !doubleWidth && charWidth > _fontWidth;
         bool smallWidth = _fixedFont && charWidth < _fontWidth;
         cr = newLine[x].rendition;
@@ -1264,7 +1268,7 @@ void TerminalDisplay::updateImage()
                 continue; // Skip trailing part of multi-col chars.
 
             bool nextIsDoubleWidth = (x+len+1 == columnsToUpdate) ? false : (newLine[x+len+1].character == 0);
-            int nxtCharWidth = fm.horizontalAdvance(QChar(newLine[x+len].character));
+            int nxtCharWidth = _fixedFont ? _fontWidth : fm.horizontalAdvance(QChar(newLine[x+len].character));
             bool nextIsbigWidth = _fixedFont && !nextIsDoubleWidth && nxtCharWidth > _fontWidth;
             bool nextIsSmallWidth = _fixedFont && newLine[x+len].character && nxtCharWidth < _fontWidth;
 
@@ -1355,8 +1359,6 @@ void TerminalDisplay::updateImage()
 
   if ( _hasBlinker && !_blinkTimer->isActive()) _blinkTimer->start( TEXT_BLINK_DELAY );
   if (!_hasBlinker && _blinkTimer->isActive()) { _blinkTimer->stop(); _blinking = false; }
-  delete[] dirtyMask;
-  delete[] disstrU;
 
 }
 
@@ -1926,7 +1928,7 @@ void TerminalDisplay::drawContents(QPainter &paint, const QRect &rect)
 
       bool lineDraw = isLineChar(_image[loc(x,y)]);
       bool doubleWidth = (_image[ qMin(loc(x,y)+1,_imageSize) ].character == 0);
-      int charWidth = fm.horizontalAdvance(QChar(c));
+      int charWidth = _fixedFont ? _fontWidth : fm.horizontalAdvance(QChar(c));
       bool bigWidth = _fixedFont && !doubleWidth && charWidth > _fontWidth;
       bool tooWide = bigWidth && charWidth >= 2 * _fontWidth;
       bool smallWidth = _fixedFont && c && charWidth < _fontWidth;
@@ -1943,7 +1945,7 @@ void TerminalDisplay::drawContents(QPainter &paint, const QRect &rect)
              _image[loc(x+len,y)].rendition == currentRendition &&
              (nxtDoubleWidth = (_image[qMin(loc(x+len,y)+1,_imageSize)].character == 0)) == doubleWidth &&
              !smallWidth &&
-             !(_fixedFont && (nxtC = _image[loc(x+len,y)].character) && (nxtCharWidth = fm.horizontalAdvance(QChar(nxtC))) < _fontWidth) &&
+             !(_fixedFont && (nxtC = _image[loc(x+len,y)].character) && (nxtCharWidth = (_fixedFont ? _fontWidth : fm.horizontalAdvance(QChar(nxtC)))) < _fontWidth) &&
              !bigWidth &&
              !(_fixedFont && !nxtDoubleWidth && nxtC && nxtCharWidth > _fontWidth) &&
              isLineChar(_image[loc(x+len,y)]) == lineDraw) // Assignment!

lib/TerminalDisplay.h

@@ -28,6 +28,7 @@
 #include <QString>
 #include <QObject>
 #include <QSize>
+#include <vector>
 
 // Konsole
 #include "Filter.h"
@@ -875,6 +876,8 @@ private slots:
                // only the area [usedLines][usedColumns] in the image contains valid data
 
     int _imageSize;
+    std::vector<wchar_t> _updateDisstrU;
+    std::vector<char>    _updateDirtyMask;
     QVector<LineProperty> _lineProperties;
 
     ColorEntry _colorTable[TABLE_COLORS];