Program: 165652
Project: 165652
Buggy source:
private Pair<Panel,Integer> addItemToList(ItemStack stack,WidgetList itemList,Pair<Panel,Integer> currentPos,int numcolumns,int labelWidth,int spacing,int slot,boolean newgroup,String groupName){
  Panel panel=currentPos.getKey();
  if (panel == null || currentPos.getValue() >= numcolumns || (newgroup && groupName != null)) {
    if (newgroup && groupName != null) {
      AbstractWidget groupLabel=new Label(mc,this).setText(groupName).setColor(ModularStorageConfiguration.groupForeground).setColor(StyleConfig.colorTextInListNormal).setHorizontalAlignment(HorizontalAlignment.ALIGN_LEFT).setFilledBackground(ModularStorageConfiguration.groupBackground).setDesiredHeight(10).setDesiredWidth(231);
      itemList.addChild(new Panel(mc,this).setLayout(new HorizontalLayout().setHorizontalMargin(2).setVerticalMargin(0)).setDesiredHeight(10).addChild(groupLabel));
    }
    panel=new Panel(mc,this).setLayout(new HorizontalLayout().setSpacing(spacing)).setDesiredHeight(12).setUserObject(new Integer(-1)).setDesiredHeight(16);
    currentPos=MutablePair.of(panel,0);
    itemList.addChild(panel);
  }
  BlockRender blockRender=new BlockRender(mc,this).setRenderItem(stack).setUserObject(new Integer(slot)).setOffsetX(-1).setOffsetY(-1);
  panel.addChild(blockRender);
  if (labelWidth > 0) {
    String displayName;
    if (labelWidth > 100) {
      displayName=typeModule.getLongLabel(stack);
    }
 else {
      displayName=typeModule.getShortLabel(stack);
    }
    AbstractWidget label=new Label(mc,this).setText(displayName).setColor(StyleConfig.colorTextInListNormal).setHorizontalAlignment(HorizontalAlignment.ALIGN_LEFT).setDesiredWidth(labelWidth).setUserObject(new Integer(-1));
    panel.addChild(label);
  }
  currentPos.setValue(currentPos.getValue() + 1);
  return currentPos;
}

Fixed version (ground truth):
private Pair<Panel,Integer> addItemToList(ItemStack stack,WidgetList itemList,Pair<Panel,Integer> currentPos,int numcolumns,int labelWidth,int spacing,int slot,boolean newgroup,String groupName){
  Panel panel=currentPos.getKey();
  if (panel == null || currentPos.getValue() >= numcolumns || (newgroup && groupName != null)) {
    if (newgroup && groupName != null) {
      AbstractWidget<?> groupLabel=new Label(mc,this).setText(groupName).setColor(ModularStorageConfiguration.groupForeground).setColor(StyleConfig.colorTextInListNormal).setHorizontalAlignment(HorizontalAlignment.ALIGN_LEFT).setFilledBackground(ModularStorageConfiguration.groupBackground).setDesiredHeight(10).setDesiredWidth(231);
      itemList.addChild(new Panel(mc,this).setLayout(new HorizontalLayout().setHorizontalMargin(2).setVerticalMargin(0)).setDesiredHeight(10).addChild(groupLabel));
    }
    panel=new Panel(mc,this).setLayout(new HorizontalLayout().setSpacing(spacing)).setDesiredHeight(12).setUserObject(new Integer(-1)).setDesiredHeight(16);
    currentPos=MutablePair.of(panel,0);
    itemList.addChild(panel);
  }
  BlockRender blockRender=new BlockRender(mc,this).setRenderItem(stack).setUserObject(new Integer(slot)).setOffsetX(-1).setOffsetY(-1);
  panel.addChild(blockRender);
  if (labelWidth > 0) {
    String displayName;
    if (labelWidth > 100) {
      displayName=typeModule.getLongLabel(stack);
    }
 else {
      displayName=typeModule.getShortLabel(stack);
    }
    AbstractWidget<?> label=new Label(mc,this).setText(displayName).setColor(StyleConfig.colorTextInListNormal).setHorizontalAlignment(HorizontalAlignment.ALIGN_LEFT).setDesiredWidth(labelWidth).setUserObject(new Integer(-1));
    panel.addChild(label);
  }
  currentPos.setValue(currentPos.getValue() + 1);
  return currentPos;
}