do not yank chat scroll when the user has scrolled up

[thedhruvhegde]

summary

every non-busy submit forced pinTopMode, which rewrote scroll intent even if the user had parked the transcript.

pin-on-send now only runs when the current intent is not already an anchor (user scrolled away).

closes #2006

test plan

• scroll up in an agent transcript, send another message, confirm the viewport stays put
• sending while already at the bottom still pins the new prompt

[greptile-apps]

Greptile Summary

This PR changes idle prompt submission so an existing scroll anchor is preserved rather than unconditionally pinning the optimistic prompt.

• Reads the conversation’s persisted scroll intent before applying pin-on-send.
• Skips pinning whenever the current mode is anchor.
• Updates the submission-test fixture with a scroll-state getter.
• The current distinction conflates user-created anchors with anchors previously created by pin-on-send.

Confidence Score: 4/5

This PR is not yet safe to merge because a subsequent idle submission can remain anchored to the previous prompt instead of pinning the newly sent prompt.

pinTopMode itself produces an anchor, and that persisted mode is not cleared when a response completes; the new blanket anchor guard therefore suppresses pinning on a realistic later submission.

Files Needing Attention: apps/emdash-desktop/src/core/features/conversations/browser/acp/acp-chat-store.ts, apps/emdash-desktop/src/renderer/tests/browser/acp-chat-store-submission.test.ts

Important Files Changed

Filename	Overview
apps/emdash-desktop/src/core/features/conversations/browser/acp/acp-chat-store.ts	Preserves anchor modes during idle submission, but conflates user-parked anchors with prior programmatic pin anchors.
apps/emdash-desktop/src/renderer/tests/browser/acp-chat-store-submission.test.ts	Adds a tail scroll getter to the fixture without testing either branch of the new scroll behavior.

Flowchart

%%{init: {'theme': 'neutral'}}%%
flowchart TD
  S[Submit prompt while idle] --> R[Read persisted scroll mode]
  R -->|tail| P[Pin new optimistic prompt]
  R -->|anchor| K[Keep existing anchor]
  A[Previous pin-on-send] -->|also produces anchor| R
  K --> O[Viewport may remain pinned to previous prompt]

Loading

Prompt To Fix All With AI

### Issue 1
apps/emdash-desktop/src/core/features/conversations/browser/acp/acp-chat-store.ts:471
**Prior anchors block new pins**

The new guard treats every `anchor` as evidence that the user scrolled away, but `pinTopMode()` also creates an anchor when a prompt is sent. Response completion does not automatically restore `tail`, so a later idle submission can leave the viewport anchored to the previous prompt instead of pinning the new optimistic prompt. The code needs to distinguish a user-parked position from a prior programmatic pin.

### Issue 2
apps/emdash-desktop/src/renderer/tests/browser/acp-chat-store-submission.test.ts:376
**Scroll branches remain untested**

The updated fixture always returns `tail`, so it never exercises the new `anchor` branch or asserts either required result: preserving an existing anchor and pinning a tail submission. Add focused cases for both states so regressions in this scroll behavior do not pass unnoticed.

---

For each issue above, determine whether it is valid and should be fixed. If so, fix it directly.

_{Reviews (1): Last reviewed commit: "do not pin the chat to a new prompt when..." | Re-trigger Greptile}

[greptile-apps]

Prior anchors block new pins

The new guard treats every anchor as evidence that the user scrolled away, but pinTopMode() also creates an anchor when a prompt is sent. Response completion does not automatically restore tail, so a later idle submission can leave the viewport anchored to the previous prompt instead of pinning the new optimistic prompt. The code needs to distinguish a user-parked position from a prior programmatic pin.

Knowledge Base Used:

• Chat state, streaming, and composition
• Agent chat experience

Prompt To Fix With AI

This is a comment left during a code review.
Path: apps/emdash-desktop/src/core/features/conversations/browser/acp/acp-chat-store.ts
Line: 471

Comment:
**Prior anchors block new pins**

The new guard treats every `anchor` as evidence that the user scrolled away, but `pinTopMode()` also creates an anchor when a prompt is sent. Response completion does not automatically restore `tail`, so a later idle submission can leave the viewport anchored to the previous prompt instead of pinning the new optimistic prompt. The code needs to distinguish a user-parked position from a prior programmatic pin.

**Knowledge Base Used:**
- [Chat state, streaming, and composition](https://app.greptile.com/emdash/-/custom-context/knowledge-base/generalaction/emdash/-/docs/chat-state-and-composition.md)
- [Agent chat experience](https://app.greptile.com/emdash/-/custom-context/knowledge-base/generalaction/emdash/-/docs/chat-experience.md)

---

For each issue above, determine whether it is valid and should be fixed. If so, fix it directly.

[greptile-apps]

Scroll branches remain untested

The updated fixture always returns tail, so it never exercises the new anchor branch or asserts either required result: preserving an existing anchor and pinning a tail submission. Add focused cases for both states so regressions in this scroll behavior do not pass unnoticed.

Prompt To Fix With AI

This is a comment left during a code review.
Path: apps/emdash-desktop/src/renderer/tests/browser/acp-chat-store-submission.test.ts
Line: 376

Comment:
**Scroll branches remain untested**

The updated fixture always returns `tail`, so it never exercises the new `anchor` branch or asserts either required result: preserving an existing anchor and pinning a tail submission. Add focused cases for both states so regressions in this scroll behavior do not pass unnoticed.

---

For each issue above, determine whether it is valid and should be fixed. If so, fix it directly.

Note: If this suggestion doesn't match your team's coding style, reply to this and let me know. I'll remember it for next time!