SelectableVisualElement.cs

using UnityEngine.UIElements;

public class SelectableVisualElement : VisualElement
{
    internal new class UxmlFactory : UxmlFactory<SelectableVisualElement> { }

    static SelectableVisualElement CurrentSelected;

    public virtual string HoverStyle { get => "selectable__hover"; }

    public virtual string SelectedStyle { get => "selectable__selected"; }

    public bool Selected { get; set; }

    public SelectableVisualElement()
    {
        RegisterCallback<MouseEnterEvent>((evt) => EnableInClassList(HoverStyle, true));
        RegisterCallback<MouseLeaveEvent>((evt) => EnableInClassList(HoverStyle, false));
        RegisterCallback<MouseDownEvent>((evt) => Select());
    }

    void Select()
    {
        if (CurrentSelected != null)
        {
            CurrentSelected.EnableInClassList(SelectedStyle, false);
            Selected = false;
            CurrentSelected = null;
        }

        EnableInClassList(SelectedStyle, true);
        CurrentSelected = this;
    }
}

A simple way to create a VisualElement that can be selected.