そしてありがとうございました。
この質問は、この古い未回答の質問に非常によく似ています: How to paint notebook-like lines as TextBox background? しかし、それは同じではありません - 正確ではありません。
メモ帳、罫線入りの紙のような背景を作成したいのですが、XAML でブラシを繰り返す方法に慣れていません。どうやって?
編集
TextBox の一部としてのソリューションは次のとおりです。
<TextBox TextBlock.LineHeight="20"
TextBlock.LineStackingStrategy="BlockLineHeight"
Padding="20,10,20,20" TextWrapping="Wrap">
<TextBox.Background>
<DrawingBrush TileMode="Tile" Stretch="None" Viewport="0,0,20,20"
ViewportUnits="Absolute" Opacity=".07">
<DrawingBrush.Drawing>
<GeometryDrawing>
<GeometryDrawing.Pen>
<Pen Brush="RoyalBlue" />
</GeometryDrawing.Pen>
<GeometryDrawing.Geometry>
<LineGeometry StartPoint="0,0" EndPoint="20,0"/>
</GeometryDrawing.Geometry>
</GeometryDrawing>
</DrawingBrush.Drawing>
</DrawingBrush>
</TextBox.Background>
Now is the time for all good men to come to the aid of their country.
Now is the time for all good men to come to the aid of their country.
Now is the time for all good men to come to the aid of their country.
Now is the time for all good men to come to the aid of their country.
Now is the time for all good men to come to the aid of their country.
</TextBox>