has the following routes 
 app.vue > a.vue > b.vue 
 a.vue is a full-screen content 
 b.vue is also a full-screen content, and less parts are shared with a, so you can ignore 
 router-view at the bottom of a.vue 
when a-> b, the content of an is above the page, but the content of b is below it
- how do I hide a content?
 - is this correct and reasonable, and is there any other way?
 
