Tried to resolve some bug

- If the user scoll from page 0 -> page 3 and navigates in that page for
some time, many fragment transactions occur and one of them may be
'removing page 0' (to save memory, I guess because it has been invisible
for some amount of time)
- So I forced to invalidate screen before scrolling
- It may get scroll performance down (and waste memory) as I made it
invalidate all screens