Visibility="{TemplateBinding IsSelected这个地方错了吧

对不起，你是对的，`Visibility`应该是根据`IsSelected`属性来切换显示隐藏。在XAML中，`{TemplateBinding}`用于从绑定到视图模型（ViewModel）的属性获取值，所以正确的语法应改为：

<!-- 判断是否选中，若选中则矩形可见 -->
<Rectangle Fill="{TemplateBinding Background}"
          Stroke="{TemplateBinding BorderBrush}"
          StrokeThickness="{TemplateBinding BorderThickness}"
          Visibility="{Binding Path=IsSelected, RelativeSource={RelativeSource Self}, Converter={StaticResource BooleanToVisibilityConverter}}" />

这里假设你已经定义了一个`BooleanToVisibilityConverter`转换器，它将`true`映射为`Visible`，`false`映射为`Collapsed`，这样当`IsSelected`为真时，矩形会显示出来，反之则隐藏。

如果你希望仅在鼠标悬停时改变元素状态，可以使用`MouseEnter`和`MouseLeave`事件：

<Rectangle onMouseEnter="Rectangle_MouseEnter"
          onMouseLeave="Rectangle_MouseLeave"
          ...>

然后在后台代码中编写对应的事件处理函数：

private void Rectangle_MouseEnter(object sender, MouseEventArgs e)
{
    // 当鼠标悬停时，改变Visibility
    var rectangle = sender as Rectangle;
    rectangle.Visibility = Visibility.Visible;
}

private void Rectangle_MouseLeave(object sender, MouseEventArgs e)
{
    // 鼠标离开时恢复默认状态
    var rectangle = sender as Rectangle;
    rectangle.Visibility = Visibility.Collapsed;
}