Copilot Chat: Add Prompt Markers to the Chat Scrollbar for Faster Navigation

Long chat sessions are hard to navigate because the scrollbar does not expose meaningful landmarks for user turns. Once a conversation grows, it becomes difficult to jump back to earlier prompts without manually scanning the transcript.

GitHub Copilot App solves this problem with markers in the scrollbar. This Feature Request is inspired by that UX solution.

I’d like VS Code chat to show prompt markers in the scrollbar so users can quickly navigate between their own requests.

## Proposed behavior

- Render markers in the chat scrollbar for user prompts only
- Do not create markers for assistant responses or system-generated turns
- Allow clicking a marker to reveal the corresponding prompt in the transcript
- Show a lightweight hover preview with the prompt text and timestamp
- Keep the marker rail useful in long conversations by downsampling when needed

## Settings

This feature should be configurable through experimental chat settings.

### `chat.scrollbarPromptMarkers.enabled`

Setting text:
`Controls whether typed scrollbar markers are shown in the Chat View transcript scrollbar.`

- Type: `boolean`
- Default: `false`
- Tags: `experimental`

### `chat.scrollbarPromptMarkers.maximumMarkers`

Setting text:
`Controls the maximum number of chat scrollbar prompt markers shown in the Chat View transcript scrollbar before markers are downsampled.`

- Type: `number`
- Minimum: `2`
- Default: `30`
- Tags: `experimental`

### `chat.scrollbarPromptMarkers.clickBehavior`

Setting text:
`Controls what happens when you click a chat scrollbar prompt marker in the transcript scrollbar.`

- Type: `string`
- Default: `revealAndFocus`
- Tags: `experimental`

Allowed values:

- `revealAndFocus`: `Reveal the target prompt and move keyboard focus to it.`
- `reveal`: `Reveal the target prompt without changing keyboard focus.`

## Why this helps

- Makes long chat sessions easier to scan
- Gives the scrollbar semantic landmarks instead of treating the transcript as an undifferentiated list
- Focuses on user prompts, which are the most useful navigation anchors. This plays very nicely with keyboard shortcuts **Chat: Previous User Prompt** (`workbench.action.chat.previousUserPrompt`) and **Chat: Next User Prompt** (`workbench.action.chat.nextUserPrompt`), which can be triggered by the user immediately after using these scrollbar markers to focus on prompt "A" and wishing then to quickly navigate to prompt "B"
- Avoids visual noise from intermediate assistant/system activity
- Fits well with

## Prior art / design direction

While the products are disparate, GitHub Copilot App has this prompt-timeline navigation pattern.

## Acceptance criteria

- Prompt markers appear in the chat scrollbar
- Hovering a marker shows prompt text and time
- Clicking a marker reveals the corresponding prompt
- Markers respect the configured click behavior
- Marker density remains manageable in long sessions through the maximum-markers setting
- Settings:
- `chat.scrollbarPromptMarkers.enabled` controls whether prompt markers are shown
- `chat.scrollbarPromptMarkers.enabled` defaults to `false`
- `chat.scrollbarPromptMarkers.maximumMarkers` controls the configured cap used for marker downsampling
- `chat.scrollbarPromptMarkers.maximumMarkers` defaults to `30`
- `chat.scrollbarPromptMarkers.maximumMarkers` does not accept values below `2`
- `chat.scrollbarPromptMarkers.clickBehavior` controls marker click behavior
- `chat.scrollbarPromptMarkers.clickBehavior` supports `revealAndFocus` and `reveal`
- `chat.scrollbarPromptMarkers.clickBehavior` defaults to `revealAndFocus`
- All three settings are exposed as experimental chat settings
- Feature does not interfere with existing VSCode keyboard shortcuts **Chat: Previous User Prompt** (`workbench.action.chat.previousUserPrompt`) and **Chat: Next User Prompt** (`workbench.action.chat.nextUserPrompt`)
- Feature is active in both VSCode and Agents windows