I'm trying to create a simple multiselect list using the example in the developer guide. There are no checkboxes displayed. I just get a list of the select options but can't select anything. If I click on any value, it is highlighted, but only one at a time.

My VF and controller are below. Help anyone? It seems this should be simple. Have spent several hours trying to get it to work.

<apex:page standardController="Planning_Object__c" extensions="PlanningObjectControllerExt" LightningStyleSheets="True">
    <apex:form >
        <apex:selectList multiselect="true" value="{!SelectedTypeOptions}" label="Object Type">
            <apex:selectOptions value="{!Types}"/>
        </apex:selectList>
    </apex:form>
</apex:page>

Controller:

public with sharing class PlanningObjectControllerExt {
     public List<SelectOption> PLOptions = new List<SelectOption>();
     public String SelectedPL{get;set;}
     public List<SelectOption> planningOptionTypes = new List<SelectOption>();
    String[] selectedTypeOptions = new String[]{};
     
    public ApexPages.StandardController controller;
    ID objectID = System.currentPagereference().getParameters().get('ID');
    //constructor
    public PlanningObjectControllerExt(ApexPages.StandardController standardController) {
        this.controller = controller;
        Schema.DescribeFieldResult fieldResult = Planning_Object__c.Type__c.getDescribe();
        List<Schema.PicklistEntry> options = fieldResult.getPickListValues();
        
        for(Schema.PicklistEntry s:options) {
            planningOptionTypes.add(new SelectOption(s.getLabel(),s.getValue()));            
        }
    } //end controller
    
    
    public List<SelectOption> getTypes() {
        return planningOptionTypes;
    }
    
    public String[] getSelectedTypeOptions() {
        return selectedTypeOptions;
    }  
    
    public void setSelectedTypeOptions(String[] selectedTypeOptions) {
        this.selectedTypeOptions = selectedTypeOptions;
    }
 
}