WPF中如何动态修改Paragraph的背景属性？

• 创建Paragraph实例时可正常设置Background属性并添加至richTextBox.Document.Blocks：

var p = new Paragraph
{
    LineHeight = 1.8,
    Background = _bgHighlight
};

var m = new Run("test string");
p.Inlines.Add(m);

richTextBox.Document.Blocks.Add(p);

• 将Paragraph插入Blocks后，直接修改其Background属性无效，尝试的代码如下：

internal static readonly SolidColorBrush _bgHighlight = new SolidColorBrush(Colors.DodgerBlue) { Opacity = .4 };

private static readonly LinearGradientBrush _bgNormalManual = new LinearGradientBrush(Colors.Wheat, Colors.Coral, new Point(0, 0), new Point(0, 1)) { Opacity = .5 };

richTextBox.PreviewMouseLeftButtonDown += (s, e) => {

    Paragraph p = null;
    if (e.OriginalSource is Paragraph x)
    {
        p = x;
    }
    else if (e.OriginalSource is Run y && y.Parent is Paragraph z)
    {
        p = z;
    }
    if (p != null)
    {
        p.Background = _bgNormalManual;
    }
};

• 尝试通过TextRange修改背景，仅能改变文本内容的背景，无法覆盖整个Paragraph的区域：

var textRange = new TextRange(p.ContentStart, p.ContentEnd);
textRange.ApplyPropertyValue(TextElement.BackgroundProperty, _bgNormal);

内容的提问来源于stack exchange，提问作者long