SliverEnsureSemantics class

A sliver that ensures its sliver child is included in the semantics tree.

This sliver ensures that its child sliver is still visited by the RenderViewport when constructing the semantics tree, and is not clipped out of the semantics tree by the RenderViewport when it is outside the current viewport and outside the cache extent.

The child sliver may still be excluded from the semantics tree if its RenderSliver does not provide a valid RenderSliver.semanticBounds. This sliver does not guarantee its child sliver is laid out.

Be mindful when positioning SliverEnsureSemantics in a CustomScrollView after slivers that build their children lazily, like SliverList. Lazy slivers might underestimate the total scrollable size (scroll extent) before the SliverEnsureSemantics widget. This inaccuracy can cause problems for assistive technologies (e.g., screen readers), which rely on a correct scroll extent to navigate properly; they might fail to scroll accurately to the content wrapped by SliverEnsureSemantics.

To avoid this potential issue and ensure the scroll extent is calculated accurately up to this sliver, it's recommended to use slivers that can determine their extent precisely beforehand. Instead of SliverList, consider using SliverFixedExtentList, SliverVariedExtentList, or SliverPrototypeExtentList. If using SliverGrid, ensure it employs a delegate such as SliverGridDelegateWithFixedCrossAxisCount or SliverGridDelegateWithMaxCrossAxisExtent. Using these alternatives guarantees that the scrollable area's size is known accurately, allowing assistive technologies to function correctly with SliverEnsureSemantics.