Setvisible(false) on QTextBlock is not adjusting the scroll bars

If user call setvisible(false) to some of the blocks (say 100 to 200), scroll bars are not adjusted automatically. 

Steps to reproduce:

1. Execute the attached project.
2. Click the hide button, and then scroll down. 
3. A Sudden jump is seen in scrollbar when we reach line 99. 

Expected result: 

The scrollbar should be adjusted so that there is smooth scrolling. 

Note: Tried many things, but no help:

1. ed->document()>markContentsDirty(0,ed>document()->characterCount());

2. ed->viewport()>update(); // shows correctly but wheel does not scroll 

3. static_cast<QPlainTextDocumentLayout *>(ed>document()>documentLayout())>requestUpdate();