Keyboard paging computation error while paging up

[Jugen]

This is from RichTextFX issue 935

Replicate

1. Run the following JavaFX application, which uses RichTextFX:

   import javafx.application.Application;
   import javafx.scene.Scene;
   import javafx.scene.control.ScrollPane;
   import javafx.stage.Stage;
   import org.fxmisc.flowless.VirtualizedScrollPane;
   import org.fxmisc.richtext.StyleClassedTextArea;
   
   public class Pagination extends Application {
     public static void main( final String[] args ) {
       launch( args );
     }
   
     @Override
     public void start( final Stage stage ) {
       final var editor = new StyleClassedTextArea( false );
       final var scrollbars = new VirtualizedScrollPane<>( editor );
   
       scrollbars.setVbarPolicy( ScrollPane.ScrollBarPolicy.ALWAYS );
       editor.setWrapText( true );
   
       stage.setScene( new Scene(scrollbars) );
       stage.show();
     }
   }

2. Copy multiple text paragraphs (e.g., https://www.lipsum.com/feed/html).
3. Paste the text 5 times (to ensure the scrollbar extends).
4. Do not resize the window (you can, but it'll mean more steps; if you did, start over):
   
5. Press Ctrl+Home to navigate to the top of the document.
6. Press Ctrl+End to navigate to the bottom of the document.
7. Press and hold Page Up to navigate upwards.

Expected Results

The view port navigates back one page for as long as the key is held, or the top is reached.

Actual Results

The computation for the new caret position goes awry in a variety of ways, depending on how much text is pasted and the view port size, resulting in one of the following behaviours:

• Nothing happens.
• View port scrolls up several times, but eventually stops, as though running into an asymptote.
• View port scrolls up a few times, then scrolls to the same page within an infinite loop.