fix: preserve TUI transcript follow during reflow

CHANGELOG.md

@@ -10,6 +10,8 @@
   `run_once` results to the calling session.
 - Keep `Send now` attached to the first visible queued prompt when synthetic
   runtime notifications are present.
+- Preserve transcript follow mode when a viewport resize or tall Shell output
+  reflow moves an already-bottom scroll position.
 - Add native PTY coverage for queued notifications, transcript paging and
   double-`Esc` cancellation.
 

src/mendcode/packages/opencode/script/session-runtime-smoke.ts

@@ -554,8 +554,8 @@ async function startTui(
     {
       cwd: PACKAGE_ROOT,
       env: ptyEnvironment(sandbox.env),
-      cols: 120,
-      rows: 40,
+      cols: Math.max(40, Number.parseInt(process.env.MENDCODE_SMOKE_COLS ?? "120", 10) || 120),
+      rows: Math.max(16, Number.parseInt(process.env.MENDCODE_SMOKE_ROWS ?? "40", 10) || 40),
       name: "xterm-256color",
     },
   )

src/mendcode/packages/opencode/src/cli/cmd/tui/routes/session/index.tsx

@@ -499,6 +499,7 @@ export function sessionUserMovedViewport(input: {
   viewportHeight: number
   lastViewportHeight: number
   followOutput?: boolean
+  atBottom?: boolean
 }) {
   const scrollDelta = input.scrollTop - input.lastScrollTop
   const scrollMoved = Math.abs(scrollDelta) > 1
@@ -510,9 +511,11 @@ export function sessionUserMovedViewport(input: {
   // A sticky/following scrollbox can move its scrollTop while layout is
   // settling after a streamed part changes height. A downward movement can
   // be that layout adjustment, but a negative delta is an unmistakable manual
-  // scroll-up gesture and must detach follow immediately.
+  // scroll-up gesture and must detach follow immediately. When sticky follow
+  // has already settled at the new bottom, however, a negative delta can be
+  // caused entirely by a larger viewport or a shorter reflowed child.
   if (layoutChanged) {
-    if (input.followOutput) return scrollDelta < -1
+    if (input.followOutput) return scrollDelta < -1 && input.atBottom !== true
     return false
   }
   return true
@@ -2584,6 +2587,7 @@ export function Session() {
       viewportHeight,
       lastViewportHeight: lastObservedViewportHeight,
       followOutput: followSessionOutput(),
+      atBottom,
     })
     const contentHeightChanged = Math.abs(scrollHeight - lastObservedScrollHeight) > 1
     const viewportHeightChanged = Math.abs(viewportHeight - lastObservedViewportHeight) > 1

src/mendcode/packages/opencode/test/cli/tui/prompt-working-start.test.ts

@@ -1966,10 +1966,40 @@ describe("resolveWorkingStartedAt", () => {
         viewportHeight: 48,
         lastViewportHeight: 48,
         followOutput: true,
+        atBottom: false,
       }),
     ).toBe(true)
   })
 
+  test("does not detach follow when a layout reflow moves an already-bottom viewport", () => {
+    for (const input of [
+      {
+        scrollTop: 180,
+        lastScrollTop: 200,
+        scrollHeight: 300,
+        lastScrollHeight: 300,
+        viewportHeight: 120,
+        lastViewportHeight: 100,
+      },
+      {
+        scrollTop: 180,
+        lastScrollTop: 200,
+        scrollHeight: 280,
+        lastScrollHeight: 300,
+        viewportHeight: 100,
+        lastViewportHeight: 100,
+      },
+    ]) {
+      expect(
+        sessionUserMovedViewport({
+          ...input,
+          followOutput: true,
+          atBottom: true,
+        }),
+      ).toBe(false)
+    }
+  })
+
   test("detaches follow when a manual scroll-up overlaps a content reflow", () => {
     expect(
       sessionUserMovedViewport({