https://learn.microsoft.com/ja-jp/dotnet/api/system.windows.controls.flowdocumentscrollviewer?view=netframework-4.8.1&viewFallbackFrom=windowsdesktop-10.0
FlowDocumentScrollViewer Class (System.Windows.Controls) | Microsoft Learn
Provides a control for viewing flow content in a continuous scrolling mode.
class systemwindowscontrolsmicrosoftlearn