diff --git a/graphcode/Sources/Features/LoopWorkspace/ArtifactorySection.swift b/graphcode/Sources/Features/LoopWorkspace/ArtifactorySection.swift
index a29683f6..9d1032fa 100644
--- a/graphcode/Sources/Features/LoopWorkspace/ArtifactorySection.swift
+++ b/graphcode/Sources/Features/LoopWorkspace/ArtifactorySection.swift
@@ -50,6 +50,10 @@ struct ArtifactorySection: View {
   /// the same thing — a fact about a person at a screen, never about the loop.
   let seenPostID: Int?
   let isFolded: Bool
+  /// How tall the scroll box may grow before it scrolls — the rail's share for the
+  /// board (`LoopWorkspaceRail.artifactoryHeightCap`), never more than
+  /// `maxScrollHeight`.
+  var maxHeight: CGFloat = ArtifactorySection.maxScrollHeight
   let onToggleFold: () -> Void
   /// Posts as "a human" — a click in the app has no `ZMX_SESSION` and no loop identity,
   /// which is exactly what a person talking to the whole graph is.
@@ -58,11 +62,10 @@ 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.
-  /// 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
-  /// the number asked for: enough to read a conversation, not so many that the rail
-  /// is nothing but the board.
+  /// The most the board's scroll box will ever be, on any window: about ten posts at
+  /// the rail's default width — enough to read a conversation, not so many that the
+  /// rail is nothing but the board. The rail hands down a smaller cap on a short
+  /// window (`LoopWorkspaceRail.artifactoryHeightCap`); this is the ceiling on that.
   static let maxScrollHeight: CGFloat = 600
 
   @State private var showsRecords = false
@@ -112,7 +115,7 @@ struct ArtifactorySection: View {
         // 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)
+        .frame(maxHeight: maxHeight)
         .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
diff --git a/graphcode/Sources/Features/LoopWorkspace/LoopWorkspaceRail.swift b/graphcode/Sources/Features/LoopWorkspace/LoopWorkspaceRail.swift
index 669f914d..7c0a0c4b 100644
--- a/graphcode/Sources/Features/LoopWorkspace/LoopWorkspaceRail.swift
+++ b/graphcode/Sources/Features/LoopWorkspace/LoopWorkspaceRail.swift
@@ -199,7 +199,32 @@ struct LoopWorkspaceRail: View {
     graph.edges.filter { $0.to == node.id }.compactMap { graph.nodes[id: $0.from] }
   }
 
+  /// The most of the rail the board may take before it scrolls, as a share of the
+  /// rail's own height. The other flexible sections — the summary, the diagram — are
+  /// greedy and yield; the board hugs its posts with `fixedSize` and does not. On a
+  /// short window a fixed 600pt cap was enough, with THIS LOOP's 118 and the summary's
+  /// 120 floor, to overflow the stack and push the foot of the rail off the bottom.
+  /// A share cannot overflow on its own, and 40% still shows a conversation.
+  static func artifactoryHeightCap(railHeight: CGFloat) -> CGFloat {
+    min(ArtifactorySection.maxScrollHeight, max(160, railHeight * 0.4))
+  }
+
   var body: some View {
+    // Measured at the outside, where a `GeometryReader` is the container and not a
+    // guess from within a scroll view — the rail's height is what the board's share
+    // is a share *of*.
+    GeometryReader { proxy in
+      stack(artifactoryCap: Self.artifactoryHeightCap(railHeight: proxy.size.height))
+    }
+    .frame(width: width)
+    .frame(maxHeight: .infinity)
+    .background(Theme.workspaceRail)
+    .overlay(alignment: .leading) {
+      Rectangle().fill(.white.opacity(0.07)).frame(width: 1)
+    }
+  }
+
+  private func stack(artifactoryCap: CGFloat) -> some View {
     VStack(alignment: .leading, spacing: 10) {
       // Above `THIS LOOP` rather than below it: what the loop is doing this second
       // outranks where it sits in the graph, and a section you have to scroll to is a
@@ -244,6 +269,7 @@ struct LoopWorkspaceRail: View {
       if ArtifactoryPresentation.hasContent(graph: graph, enabled: artifactoryEnabled) {
         ArtifactorySection(
           graph: graph, seenPostID: seenArtifactoryPostID, isFolded: isArtifactoryFolded,
+          maxHeight: artifactoryCap,
           onToggleFold: onArtifactoryFoldToggled, onPost: onArtifactoryPost)
       }
       footer
@@ -251,10 +277,6 @@ struct LoopWorkspaceRail: View {
     .padding(12)
     .frame(width: width, alignment: .leading)
     .frame(maxHeight: .infinity)
-    .background(Theme.workspaceRail)
-    .overlay(alignment: .leading) {
-      Rectangle().fill(.white.opacity(0.07)).frame(width: 1)
-    }
   }
 
   private func section<Content: View>(
diff --git a/graphcode/Tests/ArtifactoryRailShareTests.swift b/graphcode/Tests/ArtifactoryRailShareTests.swift
new file mode 100644
index 00000000..2e5bb8be
--- /dev/null
+++ b/graphcode/Tests/ArtifactoryRailShareTests.swift
@@ -0,0 +1,37 @@
+import Foundation
+import Testing
+
+@testable import graphcode
+
+/// The board's scroll box is capped by a share of the rail, so a rigid box can never be
+/// what pushes the foot of the rail off a short window.
+@Suite
+struct ArtifactoryRailShareTests {
+  @Test
+  func theBoardTakesAShareOfTheRailNotAFixedHeight() {
+    // A 900pt rail: 40% is 360, well under the 600 ceiling and well over the floor.
+    #expect(LoopWorkspaceRail.artifactoryHeightCap(railHeight: 900) == 360)
+    // A tall rail: the share would exceed the ceiling, so the ceiling wins — ten posts
+    // is enough on any window.
+    #expect(LoopWorkspaceRail.artifactoryHeightCap(railHeight: 2000) == 600)
+    #expect(ArtifactorySection.maxScrollHeight == 600)
+  }
+
+  /// A very short rail still shows a couple of posts rather than a sliver; below the
+  /// floor the rail has bigger problems than this section.
+  @Test
+  func aShortRailStillShowsSomething() {
+    #expect(LoopWorkspaceRail.artifactoryHeightCap(railHeight: 300) == 160)
+    #expect(LoopWorkspaceRail.artifactoryHeightCap(railHeight: 0) == 160)
+  }
+
+  /// The share leaves room for everything rigid above it: THIS LOOP (118) plus the
+  /// summary's floor (120) plus chrome, on a rail that fits one 1280×800 window.
+  @Test
+  func theShareLeavesRoomForTheRigidSections() {
+    let rail: CGFloat = 700
+    let board = LoopWorkspaceRail.artifactoryHeightCap(railHeight: rail)
+    let rigidAbove: CGFloat = 118 + 120 + 24 + 24 + 12 * 2
+    #expect(board + rigidAbove < rail)
+  }
+}