Artifactory rail: size the scroll box with fixedSize, not a measurement

graphcode/Sources/Features/LoopWorkspace/ArtifactorySection.swift

@@ -2,13 +2,6 @@ import ArtifactoryKit
 import GraphcodeKit
 import SwiftUI
 
-/// The measured height of the board's posts, reported up so the scroll box can size
-/// itself to them instead of to whatever space the rail happens to have spare.
-private struct ArtifactoryContentHeight: PreferenceKey {
-  static let defaultValue: CGFloat = 0
-  static func reduce(value: inout CGFloat, nextValue: () -> CGFloat) { value = nextValue() }
-}
-
 /// What the rail needs to know about the board without building a view to find out.
 enum ArtifactoryPresentation {
   /// Whether this graph's board has anything to show. Absent while empty, for the
@@ -61,8 +54,6 @@ struct ArtifactorySection: View {
   /// Whether the mirrored records are unfolded. Local and unpersisted, unlike the
   /// section's own fold: opening the receipts is a thing you do once to answer a
   /// question, not a way you prefer to read the board.
-  /// How tall the posts actually are, so the scroll box can ask for exactly that.
-  @State private var contentHeight: CGFloat = 0
   /// Past this the board scrolls rather than pushing the sections above it off the rail.
   /// Every note is on the board and reachable — this is only how much of it is on screen
   /// before the wheel takes over. About ten posts at the rail's default width, which is
@@ -104,30 +95,21 @@ struct ArtifactorySection: View {
             }
           }
           .frame(maxWidth: .infinity, alignment: .leading)
-          .background(
-            GeometryReader { proxy in
-              Color.clear.preference(key: ArtifactoryContentHeight.self, value: proxy.size.height)
-            }
-          )
         }
-        .onPreferenceChange(ArtifactoryContentHeight.self) { contentHeight = $0 }
         .scrollBounceBehavior(.basedOnSize)
         .defaultScrollAnchor(.bottom)
-        // Hug the posts, and only then scroll. A `ScrollView` is greedy: given
-        // `maxHeight: .infinity` it took the rail's whole slack, and `defaultScrollAnchor`
-        // pinned the posts to the bottom of that box while the header stayed at its top —
-        // so a board with two notes on it drew a header, a stretch of nothing, and then
-        // the notes. Measuring the content and asking for exactly that height (up to a
-        // cap) leaves no slack for the anchor to spread.
-        //
-        // `nil` until measured, never zero. This started as `min(contentHeight, cap)`
-        // with `contentHeight` at 0, and a zero-height scroll view never lays out its
-        // content — so the preference that would have grown it never fired, and the
-        // expanded section drew a header, the button, and nothing between. Unsized for
-        // the first pass, the content is laid out, the measurement lands, and the box
-        // snaps to it. The failure mode is now a brief greedy box, not an empty one.
-        .frame(height: contentHeight > 0 ? min(contentHeight, Self.maxScrollHeight) : nil)
-        .frame(maxHeight: contentHeight > 0 ? nil : Self.maxScrollHeight)
+        // Hug the posts, and only then scroll. A `ScrollView` is greedy — offered the
+        // rail's slack it takes it, and `defaultScrollAnchor(.bottom)` then pins the
+        // posts to the foot of that box with a gap between them and the header. Two
+        // attempts measured the content through a preference and sized the box to it;
+        // both mis-sized (a zero start that never laid out, then a stale reading that
+        // left the gap). This is the idiom that needs no measuring: `fixedSize`
+        // (vertical) asks the scroll view for its *ideal* height, which is its
+        // content's, and `frame(maxHeight:)` under it clamps that. The box is exactly
+        // as tall as the posts until the cap, and scrolls after — no state, nothing to
+        // go stale, nothing to fire late.
+        .frame(maxHeight: Self.maxScrollHeight)
+        .fixedSize(horizontal: false, vertical: true)
         // Outside the scroll view on purpose. Inside it the composer was one more row
         // in a list that can be taller than the rail — it could open scrolled out of
         // sight, and it moved under the pointer as posts arrived. Pinned here it is