fail to clean pseudo header when window scroll but cursor not moved 

For example, in any non-empty directory, keep pressing `<C-f>` till the end and then press `<C-b>`. We'll see the pseudo header remain there.

Since there's no window scroll event in vim,  there seems to be no better solution other than remap all `<C-f>, <C-b>, <C-d>, <C-u>, <C-e>, <C-y>` (which I definitely won't do so).