Add viewport documentation breadcrumbs (#63192)

packages/flutter/lib/src/rendering/object.dart

@@ -2421,6 +2421,11 @@ abstract class RenderObject extends AbstractNode with DiagnosticableTreeMixin im
   /// semantics tree to implement implicit accessibility scrolling on iOS where
   /// the viewport scrolls implicitly when moving the accessibility focus from
   /// a the last visible node in the viewport to the first hidden one.
+  ///
+  /// See also:
+  ///
+  /// * [RenderViewportBase.cacheExtent], used by viewports to extend their
+  ///   semantics clip beyond their approximate paint clip.
   Rect describeSemanticsClip(covariant RenderObject child) => null;
 
   // SEMANTICS
@@ -2906,6 +2911,11 @@ abstract class RenderObject extends AbstractNode with DiagnosticableTreeMixin im
   ///
   /// The `duration` parameter can be set to a non-zero value to bring the
   /// target object on screen in an animation defined by `curve`.
+  ///
+  /// See also:
+  ///
+  /// * [RenderViewportBase.showInViewport], which [RenderViewportBase] and
+  ///   [SingleChildScrollView] delegate this method to.
   void showOnScreen({
     RenderObject descendant,
     Rect rect,
@@ -3805,6 +3815,12 @@ class _SemanticsGeometry {
   /// Value for [SemanticsNode.rect].
   Rect get rect => _rect;
 
+  /// Computes values, ensuring `rect` is properly bounded by ancestor clipping rects.
+  ///
+  /// See also:
+  ///
+  /// * [RenderObject.describeSemanticsClip], typically used to determine `parentSemanticsClipRect`.
+  /// * [RenderObject.describeApproximatePaintClip], typically used to determine `parentPaintClipRect`.
   void _computeValues(Rect parentSemanticsClipRect, Rect parentPaintClipRect, List<RenderObject> ancestors) {
     assert(ancestors.length > 1);
 

packages/flutter/lib/src/rendering/viewport.dart

@@ -188,6 +188,21 @@ abstract class RenderViewportBase<ParentDataClass extends ContainerParentDataMix
        _cacheExtentStyle = cacheExtentStyle,
        _clipBehavior = clipBehavior;
 
+  /// Report the semantics of this node, for example for accessibility purposes.
+  ///
+  /// [RenderViewportBase] adds [RenderViewport.useTwoPaneSemantics] to the
+  /// provided [SemanticsConfiguration] to support children using
+  /// [RenderViewport.excludeFromScrolling].
+  ///
+  /// This method should be overridden by subclasses that have interesting
+  /// semantic information. Overriding subclasses should call
+  /// `super.describeSemanticsConfiguration(config)` to ensure
+  /// [RenderViewport.useTwoPaneSemantics] is still added to `config`.
+  ///
+  /// See also:
+  ///
+  /// * [RenderObject.describeSemanticsConfiguration], for important
+  ///   details about not mutating a [SemanticsConfiguration] out of context.
   @override
   void describeSemanticsConfiguration(SemanticsConfiguration config) {
     super.describeSemanticsConfiguration(config);
@@ -1040,6 +1055,11 @@ abstract class RenderViewportBase<ParentDataClass extends ContainerParentDataMix
   ///
   /// The `duration` parameter can be set to a non-zero value to animate the
   /// target object into the viewport with an animation defined by `curve`.
+  ///
+  /// See also:
+  ///
+  /// * [RenderObject.showOnScreen], overridden by [RenderViewportBase] and the
+  ///   renderer for [SingleChildScrollView] to delegate to this method.
   static Rect showInViewport({
     RenderObject descendant,
     Rect rect,
@@ -1188,6 +1208,11 @@ class RenderViewport extends RenderViewportBase<SliverPhysicalContainerParentDat
   /// The semantic scrolling actions and the [SemanticsNode]s of scrollable
   /// children will be attached to the inner node, which itself is a child of
   /// the outer node.
+  ///
+  /// See also:
+  ///
+  /// * [RenderViewportBase.describeSemanticsConfiguration], which adds this
+  ///   tag to its [SemanticsConfiguration].
   static const SemanticsTag useTwoPaneSemantics = SemanticsTag('RenderViewport.twoPane');
 
   /// When a top-level [SemanticsNode] below a [RenderAbstractViewport] is