When a Sliver with items is outside of the Viewport, but within the Viewport's cacheExtent, the framework should create SemanticNodes for the items even though they are out of view. However, for this to work, the Sliver's geometry must have a cacheExtent (how much space the sliver took up of the Viewport's cacheExtent) greater than 0, otherwise it is excluded.

SliverFillRemaining widgets that fall outside the viewport did not have this set and therefore were being excluded when SemanticNodes were created, even if they were within the Viewport's cacheExtent. This PR sets the cacheExtent for SliverFillRemaining widgets.

In addition, RenderSliverFillRemainingWithScrollable would get dropped from the semantic tree because it's child had a size of 0 when outside the remaining paint extent. To fix, we give the child a maxExtent of the sliver's cacheExtent if it's outside the remaining paint extent but within the viewport's cacheExtent.

Fixes #142065.

Definitions:

• RenderViewport.cacheExtent:

  /// The viewport has an area before and after the visible area to cache items
  /// that are about to become visible when the user scrolls.
  ///
  /// Items that fall in this cache area are laid out even though they are not
  /// (yet) visible on screen. The [cacheExtent] describes how many pixels
  /// the cache area extends before the leading edge and after the trailing edge
  /// of the viewport.
  ///
  /// The total extent, which the viewport will try to cover with children, is
  /// [cacheExtent] before the leading edge + extent of the main axis +
  /// [cacheExtent] after the trailing edge.
  ///
  /// The cache area is also used to implement implicit accessibility scrolling
  /// on iOS: When the accessibility focus moves from an item in the visible
  /// viewport to an invisible item in the cache area, the framework will bring
  /// that item into view with an (implicit) scroll action.

• SliverGeometry.cacheExtent:

  /// How many pixels the sliver has consumed in the
  /// [SliverConstraints.remainingCacheExtent].

• SliverContraints.remainingCacheExtent:

  /// Describes how much content the sliver should provide starting from the
  /// [cacheOrigin].
  ///
  /// Not all content in the [remainingCacheExtent] will be visible as some
  /// of it might fall into the cache area of the viewport.
  ///
  /// Each sliver should start laying out content at the [cacheOrigin] and
  /// try to provide as much content as the [remainingCacheExtent] allows.

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the Flutter Style Guide, including Features we expect every widget to implement.
• I signed the CLA.
• I listed at least one issue that this PR fixes in the description above.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is test-exempt.
• All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel on Discord.