diff --git a/src/panel/fleet-panel.ts b/src/panel/fleet-panel.ts
index 46a6142..c1998c3 100644
--- a/src/panel/fleet-panel.ts
+++ b/src/panel/fleet-panel.ts
@@ -168,7 +168,14 @@ export class FleetPanel extends Container {
         this.runTimeline = [...(this.runTimeline ?? []), ev];
         const idx = this.liveState?.append(this.renderedTimelineCount());
         this.selectedEventIndex = idx ?? null;
-        this.renderShell();
+        // #85: while the full-message overlay is open, its content comes from the stable
+        // fullMessageEvent — rebuilding the panel here (fresh SelectList per append) churns
+        // overlay scroll/selection for nothing, and nothing beneath the overlay is visible.
+        // Skip the render; closing the overlay triggers the next full render, which picks up
+        // every accumulated append. Data above stays current (runTimeline/selectedEventIndex).
+        // (refresh() — the store-mutation path — deliberately still re-renders mid-overlay:
+        // it mutates this.list, which the post-close render reads.)
+        if (!this.fullMessageEvent) this.renderShell();
       });
     }
     this.renderShell();
diff --git a/test/helpers/workflow-panel-fixture.mts b/test/helpers/workflow-panel-fixture.mts
index 223c3f6..84cb111 100644
--- a/test/helpers/workflow-panel-fixture.mts
+++ b/test/helpers/workflow-panel-fixture.mts
@@ -58,7 +58,7 @@ export function stripAnsi(s: string): string {
 
 // ── Minimal structural Theme fake ──
 
-function fakeTheme(): Theme {
+export function fakeTheme(): Theme {
   const id = (_color: string, s: string): string => s
   return {
     fg: id,
diff --git a/test/panel-overlay-churn.test.mts b/test/panel-overlay-churn.test.mts
new file mode 100644
index 0000000..91e3303
--- /dev/null
+++ b/test/panel-overlay-churn.test.mts
@@ -0,0 +1,149 @@
+// test/panel-overlay-churn.test.mts — #85 part 1: live-append churn under the
+// full-message overlay. While the overlay is open on a LIVE timeline, every appended
+// event used to rebuild the whole panel (fresh SelectList → scroll/selection reset).
+// The overlay's content comes from the stable `fullMessageEvent`, so appends must not
+// re-render while it is open. Data currency (runTimeline growth) and tail-follow of
+// the timeline itself (overlay closed) are both preserved.
+import { test } from "node:test";
+import assert from "node:assert/strict";
+import { mkdtempSync, rmSync } from "node:fs";
+import { tmpdir } from "node:os";
+import { join } from "node:path";
+import { RunLog, type MessageEvent } from "../src/runtime/run-log.ts";
+import { WorkflowRunStore } from "../src/workflows/runtime/run-store.ts";
+import { WorkflowRegistry } from "../src/workflows/registry.ts";
+import { FleetPanel } from "../src/panel/fleet-panel.ts";
+import { fakeTheme } from "./helpers/workflow-panel-fixture.mts";
+
+const RUN_ID = "fl-churn";
+
+function msg(turnIndex: number, text: string): MessageEvent {
+  return { type: "message", role: "assistant", text, turnIndex };
+}
+
+function metaEvent() {
+  return {
+    type: "run:meta" as const, runId: RUN_ID, agent: "scout", model: "test/model",
+    task: "churn fixture", startedAt: Date.now(), track: true, todoId: null,
+  };
+}
+
+interface Panelish {
+  handleInput(s: string): void;
+  messageBodyList: unknown;
+  timelineList: unknown;
+  runTimeline: unknown[] | null;
+  fullMessageEvent: unknown;
+  children: unknown[];
+}
+
+function churnPanel(): { panel: FleetPanel; panelish: Panelish; log: RunLog; cleanup: () => void } {
+  const dir = mkdtempSync(join(tmpdir(), "churn-"));
+  const log = new RunLog(dir);
+  log.append(RUN_ID, metaEvent());
+  log.append(RUN_ID, msg(0, "first report"));
+  log.append(RUN_ID, msg(1, "second report"));
+
+  const store = new WorkflowRunStore();
+  const panel = new FleetPanel({
+    theme: fakeTheme(),
+    deps: {
+      registry: new Map(),
+      runRegistry: {
+        subscribe: () => () => {},
+        get: () => undefined,
+        list: () => [],
+      } as never,
+      lock: { acquire: () => {}, release: () => {} } as never,
+      todoSync: {} as never,
+      backendRegistry: { list: () => [], get: () => undefined } as never,
+      parentModel: { provider: "", id: "" },
+      parentCwd: "",
+      lifecycleRegistry: new Map(),
+      lifecycleRuns: new Map(),
+      lifecycleDeps: {} as never,
+      workflowController: {} as never,
+      workflowStore: store,
+      workflowRegistry: new WorkflowRegistry(new Map()),
+      runLog: log,
+    },
+    onDone: () => {},
+    onNotify: () => {},
+  });
+  return {
+    panel,
+    panelish: panel as unknown as Panelish,
+    log,
+    cleanup: () => rmSync(dir, { recursive: true, force: true }),
+  };
+}
+
+test("#85: live append while full-message overlay is open does NOT rebuild the overlay", () => {
+  const { panel, panelish, log, cleanup } = churnPanel();
+  try {
+    // fleet → lifecycle → runs
+    panel.handleInput("\t");
+    panel.handleInput("\t");
+    // Enter on the run row → live timeline (run:meta only → status running)
+    panel.handleInput("\r");
+    assert.ok(panelish.timelineList, "timeline list built");
+    // Enter on the first timeline event → full-message overlay
+    panel.handleInput("\r");
+    assert.ok(panelish.fullMessageEvent, "full-message overlay open");
+    const overlayBefore = panelish.messageBodyList;
+    const childrenBefore = panelish.children.length;
+    assert.ok(overlayBefore, "overlay body list captured");
+
+    // A live append arrives while the overlay is open — the churn case.
+    log.append(RUN_ID, msg(2, "third report — arrives under the overlay"));
+
+    assert.equal(panelish.messageBodyList, overlayBefore, "overlay body list identity stable (no rebuild)");
+    assert.equal(panelish.children.length, childrenBefore, "panel children untouched (no rebuild)");
+    // Data currency is preserved even though the render was skipped:
+    // runTimeline = replay(meta + 2 msgs) + the appended msg = 4.
+    assert.equal(panelish.runTimeline?.length, 4, "runTimeline still grew (meta + 3 messages)");
+  } finally {
+    cleanup();
+  }
+});
+
+test("#85: live append with overlay CLOSED still tail-follows (timeline re-renders)", () => {
+  const { panel, panelish, log, cleanup } = churnPanel();
+  try {
+    panel.handleInput("\t");
+    panel.handleInput("\t");
+    panel.handleInput("\r"); // live timeline
+    assert.ok(panelish.timelineList, "timeline list built");
+    assert.equal(panelish.fullMessageEvent, null, "no overlay");
+    const tlBefore = panelish.timelineList;
+
+    log.append(RUN_ID, msg(2, "third report — overlay closed, tail-follow must work"));
+
+    assert.notEqual(panelish.timelineList, tlBefore, "timeline rebuilt on append (tail-follow preserved)");
+    assert.equal(panelish.runTimeline?.length, 4, "timeline grew (meta + 3 messages)");
+  } finally {
+    cleanup();
+  }
+});
+
+test("#85: closing the overlay re-renders and picks up events appended underneath", () => {
+  const { panel, panelish, log, cleanup } = churnPanel();
+  try {
+    panel.handleInput("\t");
+    panel.handleInput("\t");
+    panel.handleInput("\r"); // timeline
+    panel.handleInput("\r"); // overlay
+    log.append(RUN_ID, msg(2, "third report — under the overlay"));
+
+    panel.handleInput("\x1b"); // esc → close overlay, full rebuild
+
+    assert.equal(panelish.fullMessageEvent, null, "overlay closed");
+    assert.equal(panelish.messageBodyList, null, "overlay body list dropped");
+    assert.ok(panelish.timelineList, "timeline restored");
+    // The rebuild must reflect the appended event: timeline has 3 rows now.
+    const rendered = panel.render(120).join("\n");
+    assert.match(rendered, /third report/, "post-close render includes the under-overlay event");
+  } finally {
+    cleanup();
+  }
+});