Add prompt markers to chat transcript scrollbar

[matthewcorven]

Closes #326087

This PR adds experimental user prompt markers to the Chat View transcript scrollbar to make long chat sessions easier to scan and navigate.

What changed

• add a scrollbar overlay that renders markers for user-authored prompts in Chat View
• keep the marker set focused by excluding system-initiated turns and deduplicating retries to the latest prompt attempt
• downsample dense marker sets to a configurable cap
• support configurable click behavior: reveal only, or reveal and focus
• add hover previews showing prompt text and timestamp
• wire the same prompt-only model into prompt navigation behavior and related accessibility guidance

Settings

This PR introduces three experimental chat settings:

• chat.scrollbarPromptMarkers.enabled
  • default: false
  • controls whether prompt markers are shown in the Chat View transcript scrollbar
• chat.scrollbarPromptMarkers.maximumMarkers
  • default: 30
  • minimum: 2
  • controls the configured cap used when downsampling prompt markers
• chat.scrollbarPromptMarkers.clickBehavior
  • default: revealAndFocus
  • values: revealAndFocus, reveal
  • controls whether clicking a marker only reveals the prompt or also moves keyboard focus to it

How to test

• enable chat.scrollbarPromptMarkers.enabled
• open a long Chat View conversation with multiple user prompts
• verify prompt markers appear in the transcript scrollbar
• verify hovering a marker shows the prompt text and timestamp
• verify clicking a marker reveals the corresponding prompt
• verify chat.scrollbarPromptMarkers.clickBehavior changes click behavior as expected
• verify dense transcripts are downsampled according to chat.scrollbarPromptMarkers.maximumMarkers
• verify the feature does not appear in inline chat or quick chat

Validation

• npm run transpile-client
• focused prompt-marker unit tests
  • src/vs/workbench/contrib/chat/test/browser/actions/chatPromptNavigationActions.test.ts
  • src/vs/workbench/contrib/chat/test/browser/widget/chatScrollbarPromptMarkerController.test.ts

Notes

• the public setting key remains chat.scrollbarPromptMarkers.maximumMarkers
• internal naming was clarified so the configured max-count setting and its minimum allowed value are easier to distinguish in code

[vs-code-engineering]

📬 CODENOTIFY

The following users are being notified based on files changed in this PR:

@benibenj

Matched files:

• src/vs/base/browser/ui/list/listView.ts
• src/vs/base/browser/ui/list/listWidget.ts
• src/vs/base/browser/ui/tree/abstractTree.ts

[Copilot]

Pull request overview

This PR aims to add an experimental prompt-only marker rail to the Chat View transcript scrollbar (fixes #326087), letting users scan/navigate long sessions by their own prompts. It introduces a new ChatScrollbarPromptMarkerController, three experimental settings (chat.scrollbarPromptMarkers.enabled / .maximumMarkers / .clickBehavior), prompt-marker descriptor/downsampling logic shared with prompt navigation, plus component fixtures and unit tests.

Unfortunately, the branch appears to be built on a stale base: alongside the feature, the diff reverts a substantial amount of unrelated, recently-merged chat work. Several of these removals delete view-model members and widget methods that unchanged files still reference, so the branch will not compile as-is.

Changes:

• Adds the prompt-marker controller, settings, descriptor/downsampling logic, fixtures, and tests for the new feature.
• Reverts unrelated recent work (e.g. isTerminalCommand, getElementTop, UserToggleResize scroll-anchoring, getSelectedModelRequestOptions, turn-status-pills fixtures, chatEditingSession/checkpoint-timeline changes, and a large block of CSS), which breaks compilation of unchanged consumers.
• A new fixture host omits a required getVisiblePromptRowId override, causing a runtime crash when rendered.

Reviewed changes

Copilot reviewed 21 out of 21 changed files in this pull request and generated 6 comments.

Show a summary per file

File	Description
chat/browser/widget/chatScrollbarPromptMarkerController.ts	New controller rendering the prompt-marker rail (core feature).
chat/browser/actions/chatPromptNavigationActions.ts	Adds prompt-marker descriptor, downsampling, and click-behavior logic.
chat/common/constants.ts	Adds the three experimental settings + click-behavior enum.
chat/common/model/chatViewModel.ts	Removes isTerminalCommand/requestTimestamp — breaks unchanged consumers.
chat/browser/widget/chatListWidget.ts	Wires the controller but also removes getElementTop, UserToggleResize, and getSelectedModelRequestOptions still used elsewhere.
chat/browser/widget/media/chat.css	Adds marker styles; also deletes unrelated recent CSS.
test/browser/componentFixtures/chat/chatScrollbarPromptMarkers.fixture.ts	New fixture; host missing getVisiblePromptRowId override → runtime crash.
test/browser/componentFixtures/chat/chatWidget.fixture.ts	Adds marker fixture cases; reverts turn-status-pills fixture support.
chat/test/browser/.../chatPromptNavigationActions.test.ts, .../chatScrollbarPromptMarkerController.test.ts	Unit tests for the new descriptor/controller logic.

[matthewcorven]

Superseded by #326137.

I moved the live fork branch to agents/chat-scrollbar-markers-impl and reopened the work as a new PR because GitHub closed this PR during the branch-name transition. I replied inline on the resolved review threads here with the specific fix status; ongoing review should continue on #326137.