Program: 60001
Project: 60001
Buggy source:
@Override public void drawForeground(int mouseX,int mouseY){
  if (enableStencil) {
    glEnable(GL_STENCIL_TEST);
    glClear(GL_STENCIL_BUFFER_BIT);
    drawStencil(posX + 1,posY + 1,posX + sizeX - 1,posY + sizeY - 1,1);
  }
  FontRenderer font=getFontRenderer();
  char[] text=this.text;
  int startX=posX + 1, endX=sizeX - 1, startY=posY + 1, endY=startY + font.FONT_HEIGHT;
  for (int i=renderStart, width=0; i <= textLength; ++i) {
    boolean end=i == textLength;
    int charW=2;
    if (!end) {
      charW=font.getCharWidth(text[i]);
      if (!enableStencil && (width + charW) > endX)       break;
    }
    boolean drawCaret=i == caret && (caretCounter%=24) < 12 && isFocused();
    if (drawCaret) {
      int caretEnd=width + 2;
      if (caretInsert)       caretEnd=width + charW;
      drawModalRect(startX + width,startY - 1,startX + caretEnd,endY,(0xFF000000 & defaultCaretColor) | ~(defaultCaretColor & 0xFFFFFF));
    }
    if (!end) {
      boolean selected=i >= selectionStart & i < selectionEnd;
      if (selected)       drawModalRect(startX + width,startY,startX + width + charW,endY,selectedLineColor);
      font.drawString(String.valueOf(text[i]),startX + width,startY,selected ? selectedTextColor : textColor);
    }
    if (drawCaret) {
      int caretEnd=width + 2;
      if (caretInsert)       caretEnd=width + charW;
      GL11.glEnable(GL11.GL_BLEND);
      GL11.glBlendFunc(GL11.GL_ONE_MINUS_DST_COLOR,GL11.GL_ZERO);
      gui.drawSizedRect(startX + width,startY - 1,startX + caretEnd,endY,-1);
      GL11.glDisable(GL11.GL_BLEND);
    }
    width+=charW;
    if (width > endX)     break;
  }
  if (enableStencil)   glDisable(GL_STENCIL_TEST);
}

Fixed version (ground truth):
@Override public void drawForeground(int mouseX,int mouseY){
  if (enableStencil) {
    glEnable(GL_STENCIL_TEST);
    glClear(GL_STENCIL_BUFFER_BIT);
    drawStencil(posX + 1,posY + 1,posX + sizeX - 1,posY + sizeY - 1,1);
  }
  FontRenderer font=getFontRenderer();
  char[] text=this.text;
  int startX=posX + 1, endX=sizeX - 1, startY=posY + 1, endY=startY + font.FONT_HEIGHT;
  for (int i=renderStart, width=0; i <= textLength; ++i) {
    boolean end=i == textLength;
    int charW=2;
    if (!end) {
      charW=font.getCharWidth(text[i]);
      if (!enableStencil && (width + charW) > endX)       break;
    }
    boolean drawCaret=i == caret && (caretCounter%=24) < 12 && isFocused();
    if (drawCaret) {
      int caretEnd=width + 2;
      if (caretInsert)       caretEnd=width + charW;
      drawModalRect(startX + width,startY - 1,startX + caretEnd,endY,(0xFF000000 & defaultCaretColor) | (~defaultCaretColor & 0xFFFFFF));
    }
    if (!end) {
      boolean selected=i >= selectionStart & i < selectionEnd;
      if (selected)       drawModalRect(startX + width,startY,startX + width + charW,endY,selectedLineColor);
      font.drawString(String.valueOf(text[i]),startX + width,startY,selected ? selectedTextColor : textColor);
    }
    if (drawCaret) {
      int caretEnd=width + 2;
      if (caretInsert)       caretEnd=width + charW;
      GL11.glEnable(GL11.GL_BLEND);
      GL11.glBlendFunc(GL11.GL_ONE_MINUS_DST_COLOR,GL11.GL_ZERO);
      gui.drawSizedRect(startX + width,startY - 1,startX + caretEnd,endY,-1);
      GL11.glDisable(GL11.GL_BLEND);
    }
    width+=charW;
    if (width > endX)     break;
  }
  if (enableStencil)   glDisable(GL_STENCIL_TEST);
}