Fix OverlaySheet being too tall

Sheets with headers or footers are currently too tall, and have extra vertical space:

The problem appears to be that it's adding extra height to accommodate the header and footer (when visible), but this was unnecesary as root.contentItem.height already included their contributions to the total height.

How sheets look now:

1. Large, with header:

2. Small, with header:

3. Small, without header:

cc @teams/vdg @carlschwan