fix(chat): stop the streaming transcript floor inventing scroll space

[waleedlatif1]

Summary

• The transcript sizer floor was computed as the viewport's bottom edge (scrollTop + clientHeight), which exceeds the content height whenever the transcript is shorter than the viewport — early in a turn, or in any short chat. That invents scrollable space no content occupies.
• It stays invisible until the scroll container shrinks mid-turn (composer growing, queued-message banner appearing, window or panel resize). Then the bottom-pin scrolls into the invented space and the transcript is displaced upward until the floor drains at the end of the turn.
• Clamps the floor to the space content has actually held this turn — the max of the virtualizer's total size and the still-applied floor. Extracted to nextSizerFloor with unit tests.
• The applied-floor term is load-bearing on its own: without it, a queued message re-engaging the floor mid-drain dumps the undrained debt in a single frame, which is the jump the eased drain exists to prevent.

Type of Change

• Bug fix

Testing

Measured in Chromium against a harness rendering the real virtualizer, useAutoScroll, and the floor effect:

scenario	before	after
short transcript, composer grows mid-stream	72px upward displacement, reverts at end of turn	0px
short transcript, queued-message banner mid-stream	same displacement	0px
tall transcript	no phantom space	identical
mid-stream row shrink	floor holds, drains	identical
drain interrupted by a new turn	3px eased release	identical

509 home tests pass; typecheck and lint clean.

Note: verified in a harness that reconstructs the transcript with the real modules, not in the running app — worth a live spot-check on a short first reply.

Checklist

• Code follows project style guidelines
• Self-reviewed my changes
• Tests added/updated and passing
• No new warnings introduced
• I confirm that I have read and agree to the terms outlined in the Contributor License Agreement (CLA)

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1 Skipped Deployment

Project	Deployment	Actions	Updated (UTC)
docs	Skipped		Aug 11, 2026 3:41am

[cursor]

PR Summary

Medium Risk
Touches scroll pinning and virtualized transcript layout during streaming—user-visible and timing-sensitive—but logic is isolated with unit tests and preserves tall-transcript behavior.

Overview
Fixes phantom scroll space during streaming when the transcript is shorter than the viewport. The sizer min-height floor was tied to the viewport bottom (scrollTop + clientHeight), which can sit below real content height—so the floor invented empty scrollable area that only shows up when the scroll container shrinks mid-turn (composer growth, queued-message banner, resize), pulling the pinned transcript upward until end-of-turn drain.

nextSizerFloor centralizes the floor: a per-turn high-water mark (max of prior high water, virtualizer total size, and applied floor) caps the floor so it never exceeds space content has actually held. Mid-turn shrinks still hold via high water; undrained floor debt still carries when the floor re-engages mid-drain.

mothership-chat wires that helper, tracks high water in a ref, and resets floor state on chat switch (not when a pending chat gets its id) so a short chat doesn’t inherit another chat’s floor.

^{Reviewed by Cursor Bugbot for commit 0369620. Configure here.}

[greptile-apps]

Greptile Summary

The PR prevents short streaming transcripts from creating phantom scroll space and adds explicit floor-state cleanup when switching chats.

• Clamps the active sizer floor to the transcript’s retained high-water mark.
• Preserves undrained floor debt when a new turn interrupts the drain.
• Clears transcript-specific floor state on chat changes.
• Adds focused unit coverage for floor bounds, shrinking content, fractional offsets, and interrupted drains.

Confidence Score: 5/5

The PR appears safe to merge.

No blocking failure remains.

Important Files Changed

Filename	Overview
apps/sim/app/workspace/[workspaceId]/home/components/mothership-chat/mothership-chat.tsx	Integrates bounded floor computation, resets transcript-specific floor state on chat changes, and uses the required absolute import.
apps/sim/app/workspace/[workspaceId]/home/components/mothership-chat/sizer-floor.ts	Encapsulates the floor and high-water calculation that prevents viewport extent from inventing content space.
apps/sim/app/workspace/[workspaceId]/home/components/mothership-chat/sizer-floor.test.ts	Covers short transcripts, fractional and negative extents, content shrinkage, growth, and interrupted floor drains.

Flowchart

%%{init: {'theme': 'neutral'}}%%
flowchart TD
A[Transcript layout commit] --> B{Chat changed?}
B -->|Yes, distinct chat| C[Cancel drain and clear floor state]
B -->|No or pending chat persisted| D{Floor active?}
C --> D
D -->|Yes| E[Compute content high-water mark]
E --> F[Clamp viewport extent to high-water mark]
F --> G[Apply bounded min-height]
D -->|No| H[Drain or clear applied floor]

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_{Reviews (3): Last reviewed commit: "fix(chat): release the transcript floor ..." | Re-trigger Greptile}

[greptile-apps]

Greptile Summary

The PR extracts the transcript floor calculation and clamps it to content held during the active turn, with focused unit coverage.

• Adds nextSizerFloor and tests for short transcripts, fractional extents, row shrinkage, growth, and interrupted drains.
• Integrates a turn-level high-water mark into the streaming transcript floor and preserves undrained floor debt.
• The high-water state currently lacks a reset at chat-identity boundaries.

Confidence Score: 4/5

The PR should not merge until the floor’s high-water state is reset when the active chat changes; the import-convention issue is non-blocking.

A retained component can apply the previous tall chat’s high-water mark to a newly selected short chat for at least one layout pass, recreating artificial scroll space that then drains visibly.

Files Needing Attention: apps/sim/app/workspace/[workspaceId]/home/components/mothership-chat/mothership-chat.tsx; apps/sim/app/workspace/[workspaceId]/home/components/mothership-chat/sizer-floor.test.ts

Important Files Changed

Filename	Overview
apps/sim/app/workspace/[workspaceId]/home/components/mothership-chat/mothership-chat.tsx	Integrates content-clamped floor tracking, but the new high-water ref can cross chat boundaries and recreate phantom space.
apps/sim/app/workspace/[workspaceId]/home/components/mothership-chat/sizer-floor.ts	Provides a clear pure calculation that clamps viewport extent to content and applied-floor high-water state.
apps/sim/app/workspace/[workspaceId]/home/components/mothership-chat/sizer-floor.test.ts	Covers the pure floor arithmetic well, but does not exercise the component-level transcript reset boundary.

Flowchart

%%{init: {'theme': 'neutral'}}%%
flowchart TD
A[Tall chat is streaming] --> B[heldHighWater records tall content]
B --> C[User switches to cached short chat]
C --> D[MothershipChat instance and refs persist]
D --> E[Layout effect runs while lastRowAnimating is still true]
E --> F[nextSizerFloor retains old high-water]
F --> G[Short chat receives artificial min-height]
G --> H[Floor drains over subsequent frames]

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[waleedlatif1]

@greptile

[waleedlatif1]

@cursor review

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✅ Bugbot reviewed your changes and found no new issues!

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^{Reviewed by Cursor Bugbot for commit 0369620. Configure here.}

[waleedlatif1]

@greptile