Hi,
Somewhere int his thread :
https://www.mobileread.com/forums/sho...d.php?t=164162 , you'll find the reason and all discussions about it, begin by the end.
These line show the viewport, it useful with some modes like paper mode. In paper mode, if your page have many viewport (big zoom), when you change the page, you change of viewport. When all viewports are see, you really change of page.