[1.7.10] GUI artifacting on screen resize?

I have a gui that contains some custom buttons (mobbutton extends guibutton) and regular guibuttons.  Whenever I resize the screen the normal guibuttons seem to render in multiple places like an old windows computer locking up.  Why might this be?  I've tried resetting the button positions everytime I call draw screen or in updateScreen but that doesn't seem to help.

Here's my gui code:

package com.AndrewTackett.MobFinder.client.gui;

import java.util.ArrayList;
import java.util.List;

import com.AndrewTackett.MobFinder.EntityInfo;
import com.AndrewTackett.MobFinder.client.ClientProxy;
import com.AndrewTackett.MobFinder.client.MobPicker;
import com.AndrewTackett.MobFinder.client.MobPicker.ModMobs;
import com.AndrewTackett.MobFinder.client.MobPicker.mobInstanceContainer;

import cpw.mods.fml.relauncher.Side;
import cpw.mods.fml.relauncher.SideOnly;
import net.minecraft.client.Minecraft;
import net.minecraft.client.gui.GuiButton;
import net.minecraft.client.gui.GuiScreen;
import net.minecraft.entity.Entity;

@SideOnly(Side.CLIENT)
public class SelectionGui extends GuiScreen
{
public static final int GUI_ID = 35;
public static final int BUTTON_WIDTH = 35;
public static final int BUTTON_HEIGHT = 20;
public static final int NAME_PADDING = 3;
private static final int NAVIGATION_BUTTON_WIDTH = 78;
private static final int NEXT_ID = 1;
private static final int BACK_ID = 2;
private static final int NEXT_MOD_ID = 3;
private static final int PREVIOUS_MOD_ID = 4;
private static final int STARTING_ID = 5;
private static final int MIN_Y = 50;
private static int NAME_WIDTH = MobPicker.getMaxMobNameLength() * 6;//80;
private static int MIN_X = NAME_WIDTH + 5;
private static int MAX_Y;
private ModMobs currentMod;
private int buttonsPerPage = 1;
private int numPages = 1;
private int curPage = 1;
private int startID = 1;
private int endID = 1;
private List<GuiButton> navButtons = new ArrayList<GuiButton>();
private MobButton curButton;
private String curMobName = "";

@Override
public void initGui()
{
	super.initGui();

	currentMod = MobPicker.getCurrentMod();
	if(MobPicker.getCurrentMob()!=null)
		curMobName = EntityInfo.getMobName(MobPicker.getCurrentMob().mobClass);
	else
		curMobName = "";
	int id = STARTING_ID;

	mobInstanceContainer curMob = MobPicker.getCurrentMob();

	for(ModMobs mod : MobPicker.instance.ModContainers)
	{
		for(mobInstanceContainer mob : mod.mobs)
		{
			boolean isCurrentMob = (mob == curMob);
			mob.myButton = new MobButton(id,0,0,BUTTON_WIDTH,BUTTON_HEIGHT,EntityInfo.getMobName(mob.mobClass),isCurrentMob);
			this.buttonList.add(mob.myButton);
			id++;
		}
	}
	calculatePageValues();
	calculateStartandEndIDs();

	navButtons.add(new GuiButton(NEXT_ID, (int)(this.width * 4.0/5)-NAVIGATION_BUTTON_WIDTH,this.height - (BUTTON_HEIGHT + 5),NAVIGATION_BUTTON_WIDTH,BUTTON_HEIGHT,"Next Page"));
	navButtons.add(new GuiButton(BACK_ID, this.width/5, this.height - (BUTTON_HEIGHT + 5), NAVIGATION_BUTTON_WIDTH, BUTTON_HEIGHT,"Previous Page"));
	navButtons.add(new GuiButton(NEXT_MOD_ID,(int)(this.width * 4.0/5)-NAVIGATION_BUTTON_WIDTH,5, NAVIGATION_BUTTON_WIDTH, BUTTON_HEIGHT,"Next Mod"));
	navButtons.add(new GuiButton(PREVIOUS_MOD_ID,this.width/5,5, NAVIGATION_BUTTON_WIDTH, BUTTON_HEIGHT,"Previous Mod"));
	for(GuiButton button : navButtons)
	{
		this.buttonList.add(button);
	}
	ClientProxy.sendChatMessage("init - this.height: " + this.height + ", this.width:" + this.width);
}

@Override
public void drawScreen(int p_73863_1_, int p_73863_2_, float p_73863_3_)
    {
	setButtonPositions();
	setNavigationButtonPositions();

	this.drawDefaultBackground();

	//Draw mob toggle buttons and their labels
	for(int i=startID;i<endID;i++)
	{
		mobInstanceContainer mob = currentMod.mobs.get(i);
		mob.myButton.drawButton(this.mc, p_73863_1_, p_73863_2_);
	}

	//Draw navigation buttons
	for(int i=0;i<navButtons.size();i++)
	{
		navButtons.get(i).drawButton(this.mc, p_73863_1_, p_73863_2_);
	}

	//Draw current mod name
        this.drawCenteredString(Minecraft.getMinecraft().fontRenderer, currentMod.mod, this.width/2, 10, 0xFFFFFF);
        
        //Draw current mob name
        this.drawCenteredString(Minecraft.getMinecraft().fontRenderer, "Current mob: " + curMobName, this.width/2, 30, 0xFFFFFF);

        //Draw page indicator
        this.drawCenteredString(Minecraft.getMinecraft().fontRenderer, "(" + curPage + "/" + numPages + ")", this.width/2, this.height - 20, 0xFFFFFF);
    }

private void setButtonPositions()
{
	int x = MIN_X;
	int y = MIN_Y;
	calculateStartandEndIDs();
	for(int i=startID; i< endID;i++)
	{
		mobInstanceContainer mob = currentMod.mobs.get(i);
		mob.myButton.xPosition = x;
		mob.myButton.yPosition = y;
		y += BUTTON_HEIGHT + 5;
		if((y + BUTTON_HEIGHT) > MAX_Y)
		{
			y = MIN_Y;
			x += BUTTON_WIDTH + NAME_WIDTH;
		}
	}
}

private void calculateStartandEndIDs()
{
	startID = (curPage-1) * buttonsPerPage;
	endID = curPage * buttonsPerPage;
	if(endID > currentMod.mobs.size())
		endID = currentMod.mobs.size();
}

private void calculatePageValues()
{
	MAX_Y = this.height - (BUTTON_HEIGHT + 5);
	NAME_WIDTH = MobPicker.getMaxMobNameLength() * 6;
	MIN_X = NAME_WIDTH + NAME_PADDING;
	ClientProxy.sendChatMessage("name_width:" + NAME_WIDTH);
	int buttonAreaHeight = MAX_Y - MIN_Y;
	int numButtons = currentMod.mobs.size();
	int buttonsPerColumn = buttonAreaHeight / (BUTTON_HEIGHT + 5);
	int buttonsPerRow = (this.width) / (BUTTON_WIDTH + NAME_WIDTH + NAME_PADDING);
	buttonsPerPage = buttonsPerColumn * buttonsPerRow;
	numPages = (int) Math.ceil(numButtons * 1.0 / buttonsPerPage);
}

public void updateScreen() 
{
	//setNavigationButtonPositions();
}

private void setNavigationButtonPositions()
{
	for(int i=0;i<navButtons.size();i++)
	{
		GuiButton button = navButtons.get(i);
		switch(i)
		{
			case 0:
				button.xPosition = (int)(this.width * 4.0/5)-NAVIGATION_BUTTON_WIDTH;
				button.yPosition = this.height - (BUTTON_HEIGHT + 5);
				break;
			case 1:
				button.xPosition = this.width/5;
				button.yPosition = this.height - (BUTTON_HEIGHT + 5);
				break;
			case 2:
				button.xPosition = (int)(this.width * 4.0/5)-NAVIGATION_BUTTON_WIDTH;
				button.yPosition = 5;
				break;
			case 3:
				button.xPosition = this.width/5;
				button.yPosition = 5;
		}
	}
}

public void onGuiClosed() 
{
	locateLoadedMobInstances();
}

private void locateLoadedMobInstances()
{
	List<Entity> entityList = Minecraft.getMinecraft().theWorld.getLoadedEntityList();
	for(Entity myEntity : entityList)
	{
		MobPicker.addMobInstance(myEntity);
	}
}

@Override
protected void actionPerformed(GuiButton guibutton) 
{
	String message = "button " + guibutton.id + ":" + guibutton.displayString + " pressed!";
	ClientProxy.sendChatMessage(message);
	switch(guibutton.id)
	{
		case NEXT_ID:
			if(curPage < numPages)
				curPage++;
			else
				curPage = 1;

        	calculateStartandEndIDs();
			break;
		case BACK_ID:
			if(curPage > 1)
				curPage--;
			else
				curPage = numPages;

        	calculateStartandEndIDs();
			break;
		case NEXT_MOD_ID:
        	currentMod = MobPicker.incrementMod();
        	curPage = 1;
        	calculatePageValues();
        	calculateStartandEndIDs();
        	break;
		case PREVIOUS_MOD_ID:
        	currentMod = MobPicker.decrementMod();
        	curPage = 1;
        	calculatePageValues();
        	calculateStartandEndIDs();
        	break;
        	default:
        		MobPicker.setCurMobInstance(guibutton.id);
        		((MobButton)guibutton).toggle();
        		if(MobPicker.getCurrentMob()!=null)
        		{
        			curMobName = EntityInfo.getMobName(MobPicker.getCurrentMob().mobClass);
        			if(curButton != (MobButton)guibutton)
        			{
        				if(curButton != null)
        					curButton.toggle();
            			curButton = (MobButton) guibutton;
        			}
        		}
        		else
        		{
        			curMobName = "";
        			curButton = null;
        		}
            	break;
	}
}
}

The rest is here if that's useful: https://github.com/andrewtackett/MobFinder/tree/master/src/main/java/com/AndrewTackett/MobFinder