Overview mode in tablet mode chagnes the occlusion state for all N -1 pages, and the current discarding logic needs some improvements so that
tablet mode/overview mode works better.

1) Keep last N active windows. (at least 2)
2) Throttle reloading when there are many windows.
3) Defer discarding tabs because a user may switch overview mode quickly.

Probably not critical for m71, but I think we should do somethign like this for m72.