fix(tui): preserve detached scrolling across layout changes

packages/tui/src/tui/engine/LOCAL_CHANGES.json

@@ -49,10 +49,10 @@
     },
     {
       "path": "components/scroll-view.ts",
-      "currentSha256": "6f82fb1f426fdf81ca7058e6a1ea4a94c9ddd8f0e15d69c9020f9c66ca5411dd",
-      "changeIds": ["L035"],
-      "reason": "Allow hosts to reserve a wider always-visible scrollbar gutter while retaining at least one content column.",
-      "behaviorImpact": "The fullscreen transcript reserves three columns; other scroll views retain the default one-column gutter."
+      "currentSha256": "a2d9d5a19b40afd9d5a83f611890b592afc9764d916b24ec6090bbd5b95e21ea",
+      "changeIds": ["L035", "L043"],
+      "reason": "Allow hosts to reserve a wider always-visible scrollbar gutter while retaining at least one content column. Preserve follow state when layout changes clamp the scroll position.",
+      "behaviorImpact": "The fullscreen transcript reserves three columns; other scroll views retain the default one-column gutter. A detached viewport stays detached across layout changes until scrolling or an explicit follow request restores following."
     },
     {
       "path": "components/settings-list.ts",

packages/tui/src/tui/engine/LOCAL_CHANGES.md

@@ -177,3 +177,10 @@ Remove `L024` when the selected Pi baseline natively matches legacy-terminal `Ct
 - Minimal difference: expose a generic history-text decoder and capture/restore the existing undo extension state alongside the history draft. Plugin parsing and identity ownership stay in the product Editor.
 - Evidence: `tui-plugin-mentions.test.ts` covers repeated history navigation, identical display labels with different IDs, working-draft restoration, atomic deletion and undo.
 - Removal condition: the selected Pi baseline supports durable history decoding and draft extension state.
+
+## L043: Preserve detached scrolling across layout changes
+
+- Product contract: a detached fullscreen transcript remains detached when the viewport grows or content shrinks.
+- Minimal difference: `ScrollView.updateLayout` clamps the scroll position without changing follow state. Explicit scrolling and follow requests retain their existing behavior.
+- Evidence: `tui-scrollbar-interaction.test.ts` covers wheel detachment, viewport growth, footer/content shrink, temporarily fitting all content, subsequent output and re-arming with End.
+- Removal condition: the selected Pi baseline preserves follow state through layout clamping.

packages/tui/src/tui/engine/components/scroll-view.ts

@@ -192,9 +192,6 @@ export class ScrollView extends Container {
 		if (this.followingEnd) this.currentScrollTop = maxScrollTop;
 		else this.currentScrollTop = Math.max(0, Math.min(this.currentScrollTop, maxScrollTop));
 		if (this.currentScrollTop < maxScrollTop) this.followSuppressedAtEnd = false;
-		if (this.followEnd && this.currentScrollTop === maxScrollTop && !this.followSuppressedAtEnd) {
-			this.followingEnd = true;
-		}
 		if (this.contentHeight <= this.currentViewportHeight) this.hideTransientScrollbar();
 	}
 

packages/tui/test/unit/tui-scrollbar-interaction.test.ts

@@ -307,6 +307,59 @@ describe("Scrollbar interaction boundaries", () => {
     expect(viewportText(terminal)).toContain("composer");
   });
 
+  it.each(["footer shrink", "content shrink", "all content fitting"] as const)(
+    "keeps streaming detached after %s clamps the viewport to the end",
+    async (change) => {
+      const terminal = new VirtualTerminal(40, 15);
+      const empty = { render: () => [], invalidate() {} };
+      let lines = Array.from({ length: 60 }, (_, index) => `answer-${index}`);
+      let footerRows = 4;
+      const layout = new TuiChatLayout(terminal, {
+        surface: () => "conversation",
+        transcript: { ...empty, render: () => [...lines] },
+        welcome: empty,
+        interaction: { ...empty, isActive: () => false },
+        activity: { ...empty, render: () => Array(footerRows).fill("activity") },
+        followUp: empty,
+        composer: { ...empty, render: () => ["composer"] },
+        status: empty,
+      });
+      const tui = new TuiAltScreen(terminal);
+      screens.push(tui);
+      tui.setLayoutRoot(layout.fullscreenLayoutRoot);
+      tui.start();
+      await terminal.waitForRender();
+      terminal.sendInput("\x1b[<64;10;4M");
+      await terminal.waitForRender();
+      expect(tui.isFollowingOutput).toBe(false);
+
+      if (change === "footer shrink") footerRows = 0;
+      else lines = lines.slice(0, change === "content shrink" ? 50 : 0);
+      tui.renderNow();
+      await terminal.flush();
+      const anchor = tui.viewportTop;
+      expect(tui.isFollowingOutput).toBe(false);
+
+      for (let chunk = 0; chunk < 20; chunk++) {
+        lines.push(`streamed-${chunk}`);
+        layout.followBottom();
+        tui.renderNow();
+        await terminal.flush();
+        expect(tui.viewportTop).toBe(anchor);
+        expect(tui.isFollowingOutput).toBe(false);
+      }
+      terminal.sendInput("\x1b[F");
+      await terminal.waitForRender();
+      expect(tui.isFollowingOutput).toBe(true);
+      expect(viewportText(terminal)).toContain("streamed-19");
+      lines.push("resumed-stream");
+      layout.followBottom();
+      tui.renderNow();
+      await terminal.flush();
+      expect(viewportText(terminal)).toContain("resumed-stream");
+    },
+  );
+
   it("preserves a detached transcript position until follow-tail is explicitly re-armed", async () => {
     const terminal = new VirtualTerminal(40, 15);
     const empty = { render: () => [], invalidate() {} };
@@ -345,10 +398,29 @@ describe("Scrollbar interaction boundaries", () => {
     await terminal.waitForRender();
     expect(viewportText(terminal)).toContain("line-60");
 
+    terminal.sendInput("\x1b[<64;10;4M");
+    await terminal.waitForRender();
+    expect(tui.isFollowingOutput).toBe(false);
+
+    terminal.resize(40, 45);
+    await terminal.waitForRender();
+    const resizedTop = tui.viewportTop;
+    expect(tui.isFollowingOutput).toBe(false);
+
     content.appendLine("line-61");
     layout.followBottom();
     tui.requestRender();
     await terminal.waitForRender();
+    expect(tui.viewportTop).toBe(resizedTop);
+
+    terminal.sendInput("\x1b[F");
+    await terminal.waitForRender();
     expect(viewportText(terminal)).toContain("line-61");
+
+    content.appendLine("line-62");
+    layout.followBottom();
+    tui.requestRender();
+    await terminal.waitForRender();
+    expect(viewportText(terminal)).toContain("line-62");
   });
 });