node/Tasks: improve scroller behaviour on datastore loading

node/Tasks: improve scroller behaviour on datastore loading

when we have a fixed totalcount, the scrollbar behaves nicely,
but sometimes we do not have a fixed total, so the api will return
'+1' if there are more elments to load

when doing this, the scrollbar has a weird behaviour where it changes
size frequently the more one scrolls down.

instead, tell the grid to update its layout as soon as the store
prefetches the data, which triggers an update of the scrollbar

this still lets it jump around a little, but only once per load and
more consistent

Signed-off-by: Dominik Csapak <d.csapak@proxmox.com>