[SalesForce] apex:param not assigning value within apex:actionSupport for apex:inputCheckbox

I have a simple proof of concept that is not working as expected, and what appears to be happening is that the apex:param is not actually assigning the specified value to the controller property as defined when adding the component to the page. How can I get the assignTo attribute to work properly?

Below is the simple Visualforce page used for the proof of concept. To get it to work, you'll need to create an IsActive__c checkbox field on the Account object. The idea is that when a user clicks a checkbox, the selected Account ID is displayed at the bottom of the page. But what's happening is that the selected value is not being passed to the controller.

<apex:page controller="ActionSupportParamDemoController">
    <apex:form>
        <apex:repeat value="{!accounts}" var="account">
            <apex:inputCheckbox value="{!account.IsActive__c}">
                <apex:actionSupport event="onchange"
                                    action="{!handleAccountCheckboxChange}">
                    <apex:param id="account" name="accountId" value="{!account.Id}"
                                assignTo="{!targetAccountId}"/>
                </apex:actionSupport>
            </apex:inputCheckbox>
            {!account.Name}
        </apex:repeat>
    </apex:form>
    Selected: {!selectedAccountId}
</apex:page>

Below is the controller supporting the Visualforce page.

public class ActionSupportParamDemoController {
    private Id targetAccountId;
    public Id selectedAccountId { get; set; }

    public List<Account> getAccounts() {
        return [
            SELECT Id, Name, Website, IsActive__c
            FROM Account
        ];
    }

    public Id getTargetAccountId() {
        return targetAccountId;
    }

    public PageReference handleAccountCheckboxChange() {
        System.debug('targetAccountId: ' + targetAccountId);
        selectedAccountId = targetAccountId;
        return null;
    }

    public void setTargetAccountId(Id value) {
        targetAccountId = value;
    }
}