The (estimated) total scrollable extent that this sliver has content for.
This is the amount of scrolling the user needs to do to get from the beginning of this sliver to the end of this sliver.
The value is used to calculate the SliverConstraints.scrollOffset of all slivers in the scrollable and thus should be provided whether the sliver is currently in the viewport or not.
In a typical scrolling scenario, the scrollExtent is constant for a
sliver throughout the scrolling while paintExtent and layoutExtent
will progress from
0 when offscreen to between
0 and scrollExtent
as the sliver scrolls partially into and out of the screen and is
equal to scrollExtent while the sliver is entirely on screen. However,
these relationships can be customized to achieve more special effects.
final double scrollExtent